diff --git a/intl/icu/source/common/locmap.cpp b/intl/icu/source/common/locmap.cpp
index 6d62d8310c..66630bf63d 100644
--- a/intl/icu/source/common/locmap.cpp
+++ b/intl/icu/source/common/locmap.cpp
@@ -32,18 +32,9 @@
 #include "cmemory.h"
 #include "unicode/uloc.h"
 
-#if U_PLATFORM == U_PF_WINDOWS && defined(_MSC_VER) && (_MSC_VER >= 1500)
-/*
- * TODO: It seems like we should widen this to
- * either U_PLATFORM_USES_ONLY_WIN32_API (includes MinGW)
- * or U_PLATFORM_HAS_WIN32_API (includes MinGW and Cygwin)
- * but those use gcc and won't have defined(_MSC_VER).
- * We might need to #include some Windows header and test for some version macro from there.
- * Or call some Windows function and see what it returns.
- */
-#define USE_WINDOWS_LCID_MAPPING_API
+#if U_PLATFORM_HAS_WIN32_API && UCONFIG_USE_WINDOWS_LCID_MAPPING_API
 #include <windows.h>
-#include <winnls.h>
+#include <winnls.h> // LCIDToLocaleName and LocaleNameToLCID
 #endif
 
 /*
@@ -973,7 +964,7 @@ idCmp(const char* id1, const char* id2)
 /**
  * Searches for a Windows LCID
  *
- * @param posixid the Posix style locale id.
+ * @param posixID the Posix style locale id.
  * @param status gets set to U_ILLEGAL_ARGUMENT_ERROR when the Posix ID has
  *               no equivalent Windows LCID.
  * @return the LCID
@@ -1035,7 +1026,7 @@ getPosixID(const ILcidPosixMap *this_0, uint32_t hostID)
 //
 /////////////////////////////////////
 */
-#ifdef USE_WINDOWS_LCID_MAPPING_API
+#if U_PLATFORM_HAS_WIN32_API && UCONFIG_USE_WINDOWS_LCID_MAPPING_API
 /*
  * Various language tags needs to be changed:
  * quz -> qu
@@ -1053,6 +1044,7 @@ getPosixID(const ILcidPosixMap *this_0, uint32_t hostID)
     }
 
 #endif
+
 U_CAPI int32_t
 uprv_convertToPosix(uint32_t hostid, char *posixID, int32_t posixIDCapacity, UErrorCode* status)
 {
@@ -1061,8 +1053,10 @@ uprv_convertToPosix(uint32_t hostid, char *posixID, int32_t posixIDCapacity, UEr
     UBool bLookup = TRUE;
     const char *pPosixID = NULL;
 
-#ifdef USE_WINDOWS_LCID_MAPPING_API
-    char locName[LOCALE_NAME_MAX_LENGTH] = {};      // ICU name can't be longer than Windows name
+#if U_PLATFORM_HAS_WIN32_API && UCONFIG_USE_WINDOWS_LCID_MAPPING_API
+    static_assert(ULOC_FULLNAME_CAPACITY > LOCALE_NAME_MAX_LENGTH, "Windows locale names have smaller length than ICU locale names.");
+
+    char locName[157];  /* ULOC_FULLNAME_CAPACITY */
 
     // Note: Windows primary lang ID 0x92 in LCID is used for Central Kurdish and
     // GetLocaleInfo() maps such LCID to "ku". However, CLDR uses "ku" for
@@ -1070,47 +1064,37 @@ uprv_convertToPosix(uint32_t hostid, char *posixID, int32_t posixIDCapacity, UEr
     // use the Windows API to resolve locale ID for this specific case.
     if ((hostid & 0x3FF) != 0x92) {
         int32_t tmpLen = 0;
-        UChar windowsLocaleName[LOCALE_NAME_MAX_LENGTH];  // ULOC_FULLNAME_CAPACITY > LOCALE_NAME_MAX_LENGTH
-
+        char16_t windowsLocaleName[LOCALE_NAME_MAX_LENGTH] = {};
+#define LOCALE_SNAME                  0x0000005c
         // Note: LOCALE_ALLOW_NEUTRAL_NAMES was enabled in Windows7+, prior versions did not handle neutral (no-region) locale names.
-        tmpLen = LCIDToLocaleName(hostid, (PWSTR)windowsLocaleName, UPRV_LENGTHOF(windowsLocaleName), LOCALE_ALLOW_NEUTRAL_NAMES);
+        tmpLen = GetLocaleInfoA(hostid, LOCALE_SNAME, (LPSTR)locName, UPRV_LENGTHOF(locName));
         if (tmpLen > 1) {
-            int32_t i = 0;
-            // Only need to look up in table if have _, eg for de-de_phoneb type alternate sort.
-            bLookup = FALSE;
-            for (i = 0; i < UPRV_LENGTHOF(locName); i++)
-            {
-                locName[i] = (char)(windowsLocaleName[i]);
-
-                // Windows locale name may contain sorting variant, such as "es-ES_tradnl".
-                // In such cases, we need special mapping data found in the hardcoded table
-                // in this source file.
-                if (windowsLocaleName[i] == L'_')
-                {
-                    // Keep the base locale, without variant
-                    // TODO: Should these be mapped from _phoneb to @collation=phonebook, etc.?
-                    locName[i] = '\0';
-                    tmpLen = i;
-                    bLookup = TRUE;
-                    break;
-                }
-                else if (windowsLocaleName[i] == L'-')
-                {
-                    // Windows names use -, ICU uses _
-                    locName[i] = '_';
-                }
-                else if (windowsLocaleName[i] == L'\0')
-                {
-                    // No point in doing more work than necessary
-                    break;
-                }
+            /* Windows locale name may contain sorting variant, such as "es-ES_tradnl".
+            In such case, we need special mapping data found in the hardcoded table
+            in this source file. */
+            char *p = uprv_strchr(locName, '_');
+            if (p) {
+                /* Keep the base locale, without variant */
+                *p = 0;
+                tmpLen = uprv_strlen(locName);
             }
-            // TODO: Need to understand this better, why isn't it an alias?
+            else {
+                /* No hardcoded table lookup necessary */
+                bLookup = FALSE;
+            }
+            /* Change the tag separator from '-' to '_' */
+            p = locName;
+            while (*p) {
+                if (*p == '-') {
+                    *p = '_';
+                 }
+                p++;
+             }
             FIX_LANGUAGE_ID_TAG(locName, tmpLen);
             pPosixID = locName;
         }
     }
-#endif // USE_WINDOWS_LCID_MAPPING_API
+#endif
 
     if (bLookup) {
         const char *pCandidate = NULL;
@@ -1132,7 +1116,7 @@ uprv_convertToPosix(uint32_t hostid, char *posixID, int32_t posixIDCapacity, UEr
     }
 
     if (pPosixID) {
-        int32_t resLen = static_cast<int32_t>(uprv_strlen(pPosixID));
+        int32_t resLen = uprv_strlen(pPosixID);
         int32_t copyLen = resLen <= posixIDCapacity ? resLen : posixIDCapacity;
         uprv_memcpy(posixID, pPosixID, copyLen);
         if (resLen < posixIDCapacity) {
@@ -1162,21 +1146,16 @@ uprv_convertToPosix(uint32_t hostid, char *posixID, int32_t posixIDCapacity, UEr
 //
 /////////////////////////////////////
 */
-U_CAPI uint32_t
-uprv_convertToLCIDPlatform(const char* localeID)
+/*U_CAPI uint32_t
+uprv_convertToLCIDPlatform(const char* localeID, UErrorCode* status)
 {
-    // The purpose of this function is to leverage native platform name->lcid
+    if (U_FAILURE(*status)) {
+        return 0;
+    }
+
+    // The purpose of this function is to leverage the Windows platform name->lcid
     // conversion functionality when available.
-#ifdef USE_WINDOWS_LCID_MAPPING_API
-    DWORD nameLCIDFlags = 0;
-    UErrorCode myStatus = U_ZERO_ERROR;
-
-    // First check for a Windows name->LCID match, fall through to catch
-    // ICU special cases, but Windows may know it already.
-#if LOCALE_ALLOW_NEUTRAL_NAMES
-    nameLCIDFlags = LOCALE_ALLOW_NEUTRAL_NAMES;
-#endif /* LOCALE_ALLOW_NEUTRAL_NAMES */
-
+#if U_PLATFORM_HAS_WIN32_API && UCONFIG_USE_WINDOWS_LCID_MAPPING_API
     int32_t len;
     char collVal[ULOC_KEYWORDS_CAPACITY] = {};
     char baseName[ULOC_FULLNAME_CAPACITY] = {};
@@ -1185,8 +1164,8 @@ uprv_convertToLCIDPlatform(const char* localeID)
     // Check any for keywords.
     if (uprv_strchr(localeID, '@'))
     {
-        len = uloc_getKeywordValue(localeID, "collation", collVal, UPRV_LENGTHOF(collVal) - 1, &myStatus);
-        if (U_SUCCESS(myStatus) && len > 0)
+        len = uloc_getKeywordValue(localeID, "collation", collVal, UPRV_LENGTHOF(collVal) - 1, status);
+        if (U_SUCCESS(*status) && len > 0)
         {
             // If it contains the keyword collation, return 0 so that the LCID lookup table will be used.
             return 0;
@@ -1194,9 +1173,9 @@ uprv_convertToLCIDPlatform(const char* localeID)
         else
         {
             // If the locale ID contains keywords other than collation, just use the base name.
-            len = uloc_getBaseName(localeID, baseName, UPRV_LENGTHOF(baseName) - 1, &myStatus);
+            len = uloc_getBaseName(localeID, baseName, UPRV_LENGTHOF(baseName) - 1, status);
 
-            if (U_SUCCESS(myStatus) && len > 0)
+            if (U_SUCCESS(*status) && len > 0)
             {
                 baseName[len] = 0;
                 mylocaleID = baseName;
@@ -1206,9 +1185,9 @@ uprv_convertToLCIDPlatform(const char* localeID)
 
     char asciiBCP47Tag[LOCALE_NAME_MAX_LENGTH] = {};
     // this will change it from de_DE@collation=phonebook to de-DE-u-co-phonebk form
-    (void)uloc_toLanguageTag(mylocaleID, asciiBCP47Tag, UPRV_LENGTHOF(asciiBCP47Tag), FALSE, &myStatus);
+    (void)uloc_toLanguageTag(mylocaleID, asciiBCP47Tag, UPRV_LENGTHOF(asciiBCP47Tag), FALSE, status);
 
-    if (U_SUCCESS(myStatus))
+    if (U_SUCCESS(*status))
     {
         // Need it to be UTF-16, not 8-bit
         wchar_t bcp47Tag[LOCALE_NAME_MAX_LENGTH] = {};
@@ -1230,7 +1209,7 @@ uprv_convertToLCIDPlatform(const char* localeID)
         {
             // Ensure it's null terminated
             bcp47Tag[i] = L'\0';
-            LCID lcid = LocaleNameToLCID(bcp47Tag, nameLCIDFlags);
+            LCID lcid = LocaleNameToLCID(bcp47Tag, LOCALE_ALLOW_NEUTRAL_NAMES);
             if (lcid > 0)
             {
                 // Found LCID from windows, return that one, unless its completely ambiguous
@@ -1244,12 +1223,12 @@ uprv_convertToLCIDPlatform(const char* localeID)
         }
     }
 #else
-    (void)localeID; // Suppress unused variable warning.
-#endif /* USE_WINDOWS_LCID_MAPPING_API */
+    (void) localeID; // Suppress unused variable warning.
+#endif
 
-    // No found, or not implemented on platforms without native name->lcid conversion
+    // Nothing found, or not implemented.
     return 0;
-}
+}*/
 
 U_CAPI uint32_t
 uprv_convertToLCID(const char *langID, const char* posixID, UErrorCode* status)
diff --git a/intl/icu/source/common/locmap.h b/intl/icu/source/common/locmap.h
index bef62956e6..a250419d15 100644
--- a/intl/icu/source/common/locmap.h
+++ b/intl/icu/source/common/locmap.h
@@ -33,8 +33,8 @@
 U_CAPI int32_t uprv_convertToPosix(uint32_t hostid, char* posixID, int32_t posixIDCapacity, UErrorCode* status);
 
 /* Don't call these functions directly. Use uloc_getLCID instead. */
-U_CAPI uint32_t uprv_convertToLCIDPlatform(const char *localeID);   // Leverage platform conversion if possible
-U_CAPI uint32_t uprv_convertToLCID(const char *langID, const char* posixID, UErrorCode* status);
+//U_CAPI uint32_t uprv_convertToLCIDPlatform(const char* localeID, UErrorCode* status); // Leverage platform conversion if possible
+U_CAPI uint32_t uprv_convertToLCID(const char* langID, const char* posixID, UErrorCode* status);
 
 #endif /* LOCMAP_H */
 
diff --git a/intl/icu/source/common/putil.cpp b/intl/icu/source/common/putil.cpp
index 769309b422..1a0129684a 100644
--- a/intl/icu/source/common/putil.cpp
+++ b/intl/icu/source/common/putil.cpp
@@ -103,17 +103,6 @@
 #   include <windows.h>
 #   include "unicode/uloc.h"
 #   include "wintz.h"
-#if U_PLATFORM_HAS_WINUWP_API
-typedef PVOID LPMSG; // TODO: figure out how to get rid of this typedef
-#include <Windows.Globalization.h>
-#include <windows.system.userprofile.h>
-#include <wrl/wrappers/corewrappers.h>
-#include <wrl/client.h>
-
-using namespace ABI::Windows::Foundation;
-using namespace Microsoft::WRL;
-using namespace Microsoft::WRL::Wrappers;
-#endif
 #elif U_PLATFORM == U_PF_OS400
 #   include <float.h>
 #   include <qusec.h>       /* error code structure */
@@ -252,7 +241,6 @@ u_signBit(double d) {
 UDate fakeClock_t0 = 0; /** Time to start the clock from **/
 UDate fakeClock_dt = 0; /** Offset (fake time - real time) **/
 UBool fakeClock_set = FALSE; /** True if fake clock has spun up **/
-static UMutex fakeClockMutex = U_MUTEX_INTIALIZER;
 
 static UDate getUTCtime_real() {
     struct timeval posixTime;
@@ -261,6 +249,7 @@ static UDate getUTCtime_real() {
 }
 
 static UDate getUTCtime_fake() {
+    static UMutex fakeClockMutex = U_MUTEX_INTIALIZER;
     umtx_lock(&fakeClockMutex);
     if(!fakeClock_set) {
         UDate real = getUTCtime_real();
@@ -997,7 +986,8 @@ static char* searchForTZFile(const char* path, DefaultTZInfo* tzInfo) {
     /* Check each entry in the directory. */
     while((dirEntry = readdir(dirp)) != NULL) {
         const char* dirName = dirEntry->d_name;
-        if (uprv_strcmp(dirName, SKIP1) != 0 && uprv_strcmp(dirName, SKIP2) != 0) {
+        if (uprv_strcmp(dirName, SKIP1) != 0 && uprv_strcmp(dirName, SKIP2) != 0
+            && uprv_strcmp(TZFILE_SKIP, dirName) != 0 && uprv_strcmp(TZFILE_SKIP2, dirName) != 0) {
             /* Create a newpath with the new entry to test each entry in the directory. */
             CharString newpath(curpath, status);
             newpath.append(dirName, -1, status);
@@ -1024,7 +1014,7 @@ static char* searchForTZFile(const char* path, DefaultTZInfo* tzInfo) {
                 */
                 if (result != NULL)
                     break;
-            } else if (uprv_strcmp(TZFILE_SKIP, dirName) != 0 && uprv_strcmp(TZFILE_SKIP2, dirName) != 0) {
+            } else {
                 if(compareBinaryFiles(TZDEFAULT, newpath.data(), tzInfo)) {
                     int32_t amountToSkip = sizeof(TZZONEINFO) - 1;
                     if (amountToSkip > newpath.length()) {
@@ -1078,7 +1068,7 @@ uprv_tzname(int n)
     // the other code path returns a pointer to a heap location.
     // If we don't have a name already, then tzname wouldn't be any
     // better, so just fall back.
-    return uprv_strdup("Etc/UTC");
+    return uprv_strdup("");
 #endif // !U_TZNAME
 
 #else
@@ -1317,9 +1307,9 @@ uprv_pathIsAbsolute(const char *path)
   return FALSE;
 }
 
-/* Temporary backup setting of ICU_DATA_DIR_PREFIX_ENV_VAR
-   until some client wrapper makefiles are updated */
-#if U_PLATFORM_IS_DARWIN_BASED && TARGET_IPHONE_SIMULATOR
+/* Backup setting of ICU_DATA_DIR_PREFIX_ENV_VAR
+   (needed for some Darwin ICU build environments) */
+#if U_PLATFORM_IS_DARWIN_BASED && TARGET_OS_SIMULATOR
 # if !defined(ICU_DATA_DIR_PREFIX_ENV_VAR)
 #  define ICU_DATA_DIR_PREFIX_ENV_VAR "IPHONE_SIMULATOR_ROOT"
 # endif
@@ -1430,12 +1420,7 @@ static void U_CALLCONV dataDirectoryInitFn() {
 
     if(path==NULL) {
         /* It looks really bad, set it to something. */
-#if U_PLATFORM_HAS_WIN32_API
-        // Windows UWP will require icudtl.dat file in same directory as icuuc.dll
-        path = ".\\";
-#else
         path = "";
-#endif
     }
 
     u_setDataDirectory(path);
@@ -1633,11 +1618,7 @@ The variant cannot have dots in it.
 The 'rightmost' variant (@xxx) wins.
 The leftmost codepage (.xxx) wins.
 */
-    char *correctedPOSIXLocale = 0;
     const char* posixID = uprv_getPOSIXIDForDefaultLocale();
-    const char *p;
-    const char *q;
-    int32_t len;
 
     /* Format: (no spaces)
     ll [ _CC ] [ . MM ] [ @ VV]
@@ -1645,38 +1626,29 @@ The leftmost codepage (.xxx) wins.
       l = lang, C = ctry, M = charmap, V = variant
     */
 
-    if (gCorrectedPOSIXLocale != NULL) {
+    if (gCorrectedPOSIXLocale != nullptr) {
         return gCorrectedPOSIXLocale;
     }
 
-    if ((p = uprv_strchr(posixID, '.')) != NULL) {
-        /* assume new locale can't be larger than old one? */
-        correctedPOSIXLocale = static_cast<char *>(uprv_malloc(uprv_strlen(posixID)+1));
-        /* Exit on memory allocation error. */
-        if (correctedPOSIXLocale == NULL) {
-            return NULL;
-        }
-        uprv_strncpy(correctedPOSIXLocale, posixID, p-posixID);
-        correctedPOSIXLocale[p-posixID] = 0;
+    // Copy the ID into owned memory.
+    // Over-allocate in case we replace "@" with "__".
+    char *correctedPOSIXLocale = static_cast<char *>(uprv_malloc(uprv_strlen(posixID) + 1 + 1));
+    if (correctedPOSIXLocale == nullptr) {
+        return nullptr;
+    }
+    uprv_strcpy(correctedPOSIXLocale, posixID);
 
-        /* do not copy after the @ */
-        if ((p = uprv_strchr(correctedPOSIXLocale, '@')) != NULL) {
-            correctedPOSIXLocale[p-correctedPOSIXLocale] = 0;
+    char *limit;
+    if ((limit = uprv_strchr(correctedPOSIXLocale, '.')) != nullptr) {
+        *limit = 0;
+        if ((limit = uprv_strchr(correctedPOSIXLocale, '@')) != nullptr) {
+            *limit = 0;
         }
     }
 
     /* Note that we scan the *uncorrected* ID. */
-    if ((p = uprv_strrchr(posixID, '@')) != NULL) {
-        if (correctedPOSIXLocale == NULL) {
-            /* new locale can be 1 char longer than old one if @ -> __ */
-            correctedPOSIXLocale = static_cast<char *>(uprv_malloc(uprv_strlen(posixID)+2));
-            /* Exit on memory allocation error. */
-            if (correctedPOSIXLocale == NULL) {
-                return NULL;
-            }
-            uprv_strncpy(correctedPOSIXLocale, posixID, p-posixID);
-            correctedPOSIXLocale[p-posixID] = 0;
-        }
+    const char *p;
+    if ((p = uprv_strrchr(posixID, '@')) != nullptr) {
         p++;
 
         /* Take care of any special cases here.. */
@@ -1685,16 +1657,17 @@ The leftmost codepage (.xxx) wins.
             /* Don't worry about no__NY. In practice, it won't appear. */
         }
 
-        if (uprv_strchr(correctedPOSIXLocale,'_') == NULL) {
+        if (uprv_strchr(correctedPOSIXLocale,'_') == nullptr) {
             uprv_strcat(correctedPOSIXLocale, "__"); /* aa@b -> aa__b (note this can make the new locale 1 char longer) */
         }
         else {
             uprv_strcat(correctedPOSIXLocale, "_"); /* aa_CC@b -> aa_CC_b */
         }
 
-        if ((q = uprv_strchr(p, '.')) != NULL) {
+        const char *q;
+        if ((q = uprv_strchr(p, '.')) != nullptr) {
             /* How big will the resulting string be? */
-            len = (int32_t)(uprv_strlen(correctedPOSIXLocale) + (q-p));
+            int32_t len = (int32_t)(uprv_strlen(correctedPOSIXLocale) + (q-p));
             uprv_strncat(correctedPOSIXLocale, p, q-p);
             correctedPOSIXLocale[len] = 0;
         }
@@ -1710,28 +1683,15 @@ The leftmost codepage (.xxx) wins.
          */
     }
 
-    /* Was a correction made? */
-    if (correctedPOSIXLocale != NULL) {
-        posixID = correctedPOSIXLocale;
-    }
-    else {
-        /* copy it, just in case the original pointer goes away.  See j2395 */
-        correctedPOSIXLocale = (char *)uprv_malloc(uprv_strlen(posixID) + 1);
-        /* Exit on memory allocation error. */
-        if (correctedPOSIXLocale == NULL) {
-            return NULL;
-        }
-        posixID = uprv_strcpy(correctedPOSIXLocale, posixID);
-    }
-
-    if (gCorrectedPOSIXLocale == NULL) {
+    if (gCorrectedPOSIXLocale == nullptr) {
         gCorrectedPOSIXLocale = correctedPOSIXLocale;
         gCorrectedPOSIXLocaleHeapAllocated = true;
         ucln_common_registerCleanup(UCLN_COMMON_PUTIL, putil_cleanup);
-        correctedPOSIXLocale = NULL;
+        correctedPOSIXLocale = nullptr;
     }
+    posixID = gCorrectedPOSIXLocale;
 
-    if (correctedPOSIXLocale != NULL) {  /* Was already set - clean up. */
+    if (correctedPOSIXLocale != nullptr) {  /* Was already set - clean up. */
         uprv_free(correctedPOSIXLocale);
     }
 
@@ -1747,58 +1707,16 @@ The leftmost codepage (.xxx) wins.
         return gCorrectedPOSIXLocale;
     }
 
-    // No cached value, need to determine the current value
-    static WCHAR windowsLocale[LOCALE_NAME_MAX_LENGTH] = {};
-    int length = GetLocaleInfoEx(LOCALE_NAME_USER_DEFAULT, LOCALE_SNAME, windowsLocale, LOCALE_NAME_MAX_LENGTH);
-
-    // Now we should have a Windows locale name that needs converted to the POSIX style.
-    if (length > 0) // If length is 0, then the GetLocaleInfoEx failed.
-    {
-        // First we need to go from UTF-16 to char (and also convert from _ to - while we're at it.)
-        char modifiedWindowsLocale[LOCALE_NAME_MAX_LENGTH] = {};
-
-        int32_t i;
-        for (i = 0; i < UPRV_LENGTHOF(modifiedWindowsLocale); i++)
-        {
-            if (windowsLocale[i] == '_')
-            {
-                modifiedWindowsLocale[i] = '-';
-            }
-            else
-            {
-                modifiedWindowsLocale[i] = static_cast<char>(windowsLocale[i]);
-            }
-
-            if (modifiedWindowsLocale[i] == '\0')
-            {
-                break;
-            }
-        }
-
-        if (i >= UPRV_LENGTHOF(modifiedWindowsLocale))
-        {
-            // Ran out of room, can't really happen, maybe we'll be lucky about a matching
-            // locale when tags are dropped
-            modifiedWindowsLocale[UPRV_LENGTHOF(modifiedWindowsLocale) - 1] = '\0';
-        }
-
-        // Now normalize the resulting name
-        correctedPOSIXLocale = static_cast<char *>(uprv_malloc(POSIX_LOCALE_CAPACITY + 1));
-        /* TODO: Should we just exit on memory allocation failure? */
-        if (correctedPOSIXLocale)
-        {
-            int32_t posixLen = uloc_canonicalize(modifiedWindowsLocale, correctedPOSIXLocale, POSIX_LOCALE_CAPACITY, &status);
-            if (U_SUCCESS(status))
-            {
-                *(correctedPOSIXLocale + posixLen) = 0;
-                gCorrectedPOSIXLocale = correctedPOSIXLocale;
-                gCorrectedPOSIXLocaleHeapAllocated = true;
-                ucln_common_registerCleanup(UCLN_COMMON_PUTIL, putil_cleanup);
-            }
-            else
-            {
-                uprv_free(correctedPOSIXLocale);
-            }
+    LCID id = GetThreadLocale();
+    correctedPOSIXLocale = static_cast<char *>(uprv_malloc(POSIX_LOCALE_CAPACITY + 1));
+    if (correctedPOSIXLocale) {
+        int32_t posixLen = uprv_convertToPosix(id, correctedPOSIXLocale, POSIX_LOCALE_CAPACITY, &status);
+        if (U_SUCCESS(status)) {
+            *(correctedPOSIXLocale + posixLen) = 0;
+            gCorrectedPOSIXLocale = correctedPOSIXLocale;
+            ucln_common_registerCleanup(UCLN_COMMON_PUTIL, putil_cleanup);
+        } else {
+            uprv_free(correctedPOSIXLocale);
         }
     }
 
diff --git a/intl/icu/source/common/uloc.cpp b/intl/icu/source/common/uloc.cpp
index 81b6e0f68a..418c0772d3 100644
--- a/intl/icu/source/common/uloc.cpp
+++ b/intl/icu/source/common/uloc.cpp
@@ -457,8 +457,6 @@ NULL
 typedef struct CanonicalizationMap {
     const char *id;          /* input ID */
     const char *canonicalID; /* canonicalized output ID */
-    const char *keyword;     /* keyword, or NULL if none */
-    const char *value;       /* keyword value, or NULL if kw==NULL */
 } CanonicalizationMap;
 
 /**
@@ -466,64 +464,16 @@ typedef struct CanonicalizationMap {
  * different semantic kinds of transformations.
  */
 static const CanonicalizationMap CANONICALIZE_MAP[] = {
-    { "",               "en_US_POSIX", NULL, NULL }, /* .NET name */
-    { "c",              "en_US_POSIX", NULL, NULL }, /* POSIX name */
-    { "posix",          "en_US_POSIX", NULL, NULL }, /* POSIX name (alias of C) */
-    { "art_LOJBAN",     "jbo", NULL, NULL }, /* registered name */
-    { "az_AZ_CYRL",     "az_Cyrl_AZ", NULL, NULL }, /* .NET name */
-    { "az_AZ_LATN",     "az_Latn_AZ", NULL, NULL }, /* .NET name */
-    { "ca_ES_PREEURO",  "ca_ES", "currency", "ESP" },
-    { "de__PHONEBOOK",  "de", "collation", "phonebook" }, /* Old ICU name */
-    { "de_AT_PREEURO",  "de_AT", "currency", "ATS" },
-    { "de_DE_PREEURO",  "de_DE", "currency", "DEM" },
-    { "de_LU_PREEURO",  "de_LU", "currency", "LUF" },
-    { "el_GR_PREEURO",  "el_GR", "currency", "GRD" },
-    { "en_BE_PREEURO",  "en_BE", "currency", "BEF" },
-    { "en_IE_PREEURO",  "en_IE", "currency", "IEP" },
-    { "es__TRADITIONAL", "es", "collation", "traditional" }, /* Old ICU name */
-    { "es_ES_PREEURO",  "es_ES", "currency", "ESP" },
-    { "eu_ES_PREEURO",  "eu_ES", "currency", "ESP" },
-    { "fi_FI_PREEURO",  "fi_FI", "currency", "FIM" },
-    { "fr_BE_PREEURO",  "fr_BE", "currency", "BEF" },
-    { "fr_FR_PREEURO",  "fr_FR", "currency", "FRF" },
-    { "fr_LU_PREEURO",  "fr_LU", "currency", "LUF" },
-    { "ga_IE_PREEURO",  "ga_IE", "currency", "IEP" },
-    { "gl_ES_PREEURO",  "gl_ES", "currency", "ESP" },
-    { "hi__DIRECT",     "hi", "collation", "direct" }, /* Old ICU name */
-    { "it_IT_PREEURO",  "it_IT", "currency", "ITL" },
-    { "ja_JP_TRADITIONAL", "ja_JP", "calendar", "japanese" }, /* Old ICU name */
-    { "nb_NO_NY",       "nn_NO", NULL, NULL },  /* "markus said this was ok" :-) */
-    { "nl_BE_PREEURO",  "nl_BE", "currency", "BEF" },
-    { "nl_NL_PREEURO",  "nl_NL", "currency", "NLG" },
-    { "pt_PT_PREEURO",  "pt_PT", "currency", "PTE" },
-    { "sr_SP_CYRL",     "sr_Cyrl_RS", NULL, NULL }, /* .NET name */
-    { "sr_SP_LATN",     "sr_Latn_RS", NULL, NULL }, /* .NET name */
-    { "sr_YU_CYRILLIC", "sr_Cyrl_RS", NULL, NULL }, /* Linux name */
-    { "th_TH_TRADITIONAL", "th_TH", "calendar", "buddhist" }, /* Old ICU name */
-    { "uz_UZ_CYRILLIC", "uz_Cyrl_UZ", NULL, NULL }, /* Linux name */
-    { "uz_UZ_CYRL",     "uz_Cyrl_UZ", NULL, NULL }, /* .NET name */
-    { "uz_UZ_LATN",     "uz_Latn_UZ", NULL, NULL }, /* .NET name */
-    { "zh_CHS",         "zh_Hans", NULL, NULL }, /* .NET name */
-    { "zh_CHT",         "zh_Hant", NULL, NULL }, /* .NET name */
-    { "zh_GAN",         "gan", NULL, NULL }, /* registered name */
-    { "zh_GUOYU",       "zh", NULL, NULL }, /* registered name */
-    { "zh_HAKKA",       "hak", NULL, NULL }, /* registered name */
-    { "zh_MIN_NAN",     "nan", NULL, NULL }, /* registered name */
-    { "zh_WUU",         "wuu", NULL, NULL }, /* registered name */
-    { "zh_XIANG",       "hsn", NULL, NULL }, /* registered name */
-    { "zh_YUE",         "yue", NULL, NULL }, /* registered name */
-};
-
-typedef struct VariantMap {
-    const char *variant;          /* input ID */
-    const char *keyword;     /* keyword, or NULL if none */
-    const char *value;       /* keyword value, or NULL if kw==NULL */
-} VariantMap;
-
-static const VariantMap VARIANT_MAP[] = {
-    { "EURO",   "currency", "EUR" },
-    { "PINYIN", "collation", "pinyin" }, /* Solaris variant */
-    { "STROKE", "collation", "stroke" }  /* Solaris variant */
+    { "art_LOJBAN",     "jbo" }, /* registered name */
+    { "hy__AREVELA",    "hy" }, /* Registered IANA variant */
+    { "hy__AREVMDA",    "hyw" }, /* Registered IANA variant */
+    { "zh_GAN",         "gan" }, /* registered name */
+    { "zh_GUOYU",       "zh" }, /* registered name */
+    { "zh_HAKKA",       "hak" }, /* registered name */
+    { "zh_MIN_NAN",     "nan" }, /* registered name */
+    { "zh_WUU",         "wuu" }, /* registered name */
+    { "zh_XIANG",       "hsn" }, /* registered name */
+    { "zh_YUE",         "yue" }, /* registered name */
 };
 
 /* ### BCP47 Conversion *******************************************/
@@ -643,20 +593,12 @@ compareKeywordStructs(const void * /*context*/, const void *left, const void *ri
     return uprv_strcmp(leftString, rightString);
 }
 
-/**
- * Both addKeyword and addValue must already be in canonical form.
- * Either both addKeyword and addValue are NULL, or neither is NULL.
- * If they are not NULL they must be zero terminated.
- * If addKeyword is not NULL is must have length small enough to fit in KeywordStruct.keyword.
- */
 static int32_t
 _getKeywords(const char *localeID,
              char prev,
              char *keywords, int32_t keywordCapacity,
              char *values, int32_t valuesCapacity, int32_t *valLen,
              UBool valuesToo,
-             const char* addKeyword,
-             const char* addValue,
              UErrorCode *status)
 {
     KeywordStruct keywordList[ULOC_MAX_NO_KEYWORDS];
@@ -755,33 +697,6 @@ _getKeywords(const char *localeID,
             }
         } while(pos);
 
-        /* Handle addKeyword/addValue. */
-        if (addKeyword != NULL) {
-            UBool duplicate = FALSE;
-            U_ASSERT(addValue != NULL);
-            /* Search for duplicate; if found, do nothing. Explicit keyword
-               overrides addKeyword. */
-            for (j=0; j<numKeywords; ++j) {
-                if (uprv_strcmp(keywordList[j].keyword, addKeyword) == 0) {
-                    duplicate = TRUE;
-                    break;
-                }
-            }
-            if (!duplicate) {
-                if (numKeywords == maxKeywords) {
-                    *status = U_INTERNAL_PROGRAM_ERROR;
-                    return 0;
-                }
-                uprv_strcpy(keywordList[numKeywords].keyword, addKeyword);
-                keywordList[numKeywords].keywordLen = (int32_t)uprv_strlen(addKeyword);
-                keywordList[numKeywords].valueStart = addValue;
-                keywordList[numKeywords].valueLen = (int32_t)uprv_strlen(addValue);
-                ++numKeywords;
-            }
-        } else {
-            U_ASSERT(addValue == NULL);
-        }
-
         /* now we have a list of keywords */
         /* we need to sort it */
         uprv_sortArray(keywordList, numKeywords, sizeof(KeywordStruct), compareKeywordStructs, NULL, FALSE, status);
@@ -839,7 +754,7 @@ locale_getKeywords(const char *localeID,
                    UErrorCode *status) {
     return _getKeywords(localeID, prev, keywords, keywordCapacity,
                         values, valuesCapacity, valLen, valuesToo,
-                        NULL, NULL, status);
+                        status);
 }
 
 U_CAPI int32_t U_EXPORT2
@@ -1188,20 +1103,6 @@ uloc_setKeywordValue(const char* keywordName,
  */
 #define _isTerminator(a)  ((a==0)||(a=='.')||(a=='@'))
 
-static char* _strnchr(const char* str, int32_t len, char c) {
-    U_ASSERT(str != 0 && len >= 0);
-    while (len-- != 0) {
-        char d = *str;
-        if (d == c) {
-            return (char*) str;
-        } else if (d == 0) {
-            break;
-        }
-        ++str;
-    }
-    return NULL;
-}
-
 /**
  * Lookup 'key' in the array 'list'.  The array 'list' should contain
  * a NULL entry, followed by more entries, and a second NULL entry.
@@ -1279,6 +1180,16 @@ ulocimp_getLanguage(const char *localeID,
     int32_t offset;
     char lang[4]={ 0, 0, 0, 0 }; /* temporary buffer to hold language code for searching */
 
+    if (uprv_stricmp(localeID, "root") == 0) {
+        localeID += 4;
+    } else if (uprv_strnicmp(localeID, "und", 3) == 0 &&
+               (localeID[3] == '\0' ||
+                localeID[3] == '-' ||
+                localeID[3] == '_' ||
+                localeID[3] == '@')) {
+        localeID += 3;
+    }
+
     /* if it starts with i- or x- then copy that prefix */
     if(_isIDPrefix(localeID)) {
         if(i<languageCapacity) {
@@ -1476,50 +1387,6 @@ _getVariant(const char *localeID,
     return _getVariantEx(localeID, prev, variant, variantCapacity, FALSE);
 }
 
-/**
- * Delete ALL instances of a variant from the given list of one or
- * more variants.  Example: "FOO_EURO_BAR_EURO" => "FOO_BAR".
- * @param variants the source string of one or more variants,
- * separated by '_'.  This will be MODIFIED IN PLACE.  Not zero
- * terminated; if it is, trailing zero will NOT be maintained.
- * @param variantsLen length of variants
- * @param toDelete variant to delete, without separators, e.g.  "EURO"
- * or "PREEURO"; not zero terminated
- * @param toDeleteLen length of toDelete
- * @return number of characters deleted from variants
- */
-static int32_t
-_deleteVariant(char* variants, int32_t variantsLen,
-               const char* toDelete, int32_t toDeleteLen)
-{
-    int32_t delta = 0; /* number of chars deleted */
-    for (;;) {
-        UBool flag = FALSE;
-        if (variantsLen < toDeleteLen) {
-            return delta;
-        }
-        if (uprv_strncmp(variants, toDelete, toDeleteLen) == 0 &&
-            (variantsLen == toDeleteLen ||
-             (flag=(variants[toDeleteLen] == '_')) != 0))
-        {
-            int32_t d = toDeleteLen + (flag?1:0);
-            variantsLen -= d;
-            delta += d;
-            if (variantsLen > 0) {
-                uprv_memmove(variants, variants+d, variantsLen);
-            }
-        } else {
-            char* p = _strnchr(variants, variantsLen, '_');
-            if (p == NULL) {
-                return delta;
-            }
-            ++p;
-            variantsLen -= (int32_t)(p - variants);
-            variants = p;
-        }
-    }
-}
-
 /* Keyword enumeration */
 
 typedef struct UKeywordsContext {
@@ -1698,8 +1565,6 @@ _canonicalize(const char* localeID,
     const char* tmpLocaleID;
     const char* keywordAssign = NULL;
     const char* separatorIndicator = NULL;
-    const char* addKeyword = NULL;
-    const char* addValue = NULL;
     char* name;
     char* variant = NULL; /* pointer into name, or NULL */
 
@@ -1738,7 +1603,7 @@ _canonicalize(const char* localeID,
         len = (int32_t)uprv_strlen(d);
 
         if (name != NULL) {
-            uprv_strncpy(name, d, len);
+            uprv_memcpy(name, d, len);
         }
     } else if(_isIDSeparator(*tmpLocaleID)) {
         const char *scriptID;
@@ -1864,27 +1729,6 @@ _canonicalize(const char* localeID,
             }
         }
 
-        /* Handle generic variants first */
-        if (variant) {
-            for (j=0; j<UPRV_LENGTHOF(VARIANT_MAP); j++) {
-                const char* variantToCompare = VARIANT_MAP[j].variant;
-                int32_t n = (int32_t)uprv_strlen(variantToCompare);
-                int32_t variantLen = _deleteVariant(variant, uprv_min(variantSize, (nameCapacity-len)), variantToCompare, n);
-                len -= variantLen;
-                if (variantLen > 0) {
-                    if (len > 0 && name[len-1] == '_') { /* delete trailing '_' */
-                        --len;
-                    }
-                    addKeyword = VARIANT_MAP[j].keyword;
-                    addValue = VARIANT_MAP[j].value;
-                    break;
-                }
-            }
-            if (len > 0 && len <= nameCapacity && name[len-1] == '_') { /* delete trailing '_' */
-                --len;
-            }
-        }
-
         /* Look up the ID in the canonicalization map */
         for (j=0; j<UPRV_LENGTHOF(CANONICALIZE_MAP); j++) {
             const char* id = CANONICALIZE_MAP[j].id;
@@ -1894,10 +1738,6 @@ _canonicalize(const char* localeID,
                     break; /* Don't remap "" if keywords present */
                 }
                 len = _copyCount(name, nameCapacity, CANONICALIZE_MAP[j].canonicalID);
-                if (CANONICALIZE_MAP[j].keyword) {
-                    addKeyword = CANONICALIZE_MAP[j].keyword;
-                    addValue = CANONICALIZE_MAP[j].value;
-                }
                 break;
             }
         }
@@ -1912,14 +1752,7 @@ _canonicalize(const char* localeID,
             ++len;
             ++fieldCount;
             len += _getKeywords(tmpLocaleID+1, '@', (len<nameCapacity ? name+len : NULL), nameCapacity-len,
-                                NULL, 0, NULL, TRUE, addKeyword, addValue, err);
-        } else if (addKeyword != NULL) {
-            U_ASSERT(addValue != NULL && len < nameCapacity);
-            /* inelegant but works -- later make _getKeywords do this? */
-            len += _copyCount(name+len, nameCapacity-len, "@");
-            len += _copyCount(name+len, nameCapacity-len, addKeyword);
-            len += _copyCount(name+len, nameCapacity-len, "=");
-            len += _copyCount(name+len, nameCapacity-len, addValue);
+                                NULL, 0, NULL, TRUE, err);
         }
     }
 
@@ -1954,9 +1787,16 @@ uloc_getParent(const char*    localeID,
         i=0;
     }
 
-    if(i>0 && parent != localeID) {
-        uprv_memcpy(parent, localeID, uprv_min(i, parentCapacity));
+    if (i > 0) {
+        if (uprv_strnicmp(localeID, "und_", 4) == 0) {
+            localeID += 3;
+            i -= 3;
+            uprv_memmove(parent, localeID, uprv_min(i, parentCapacity));
+        } else if (parent != localeID) {
+            uprv_memcpy(parent, localeID, uprv_min(i, parentCapacity));
+        }
     }
+
     return u_terminateChars(parent, parentCapacity, i, err);
 }
 
@@ -2172,23 +2012,14 @@ uloc_getLCID(const char* localeID)
 {
     UErrorCode status = U_ZERO_ERROR;
     char       langID[ULOC_FULLNAME_CAPACITY];
-    uint32_t   lcid = 0;
 
     /* Check for incomplete id. */
     if (!localeID || uprv_strlen(localeID) < 2) {
         return 0;
     }
 
-    // Attempt platform lookup if available
-    lcid = uprv_convertToLCIDPlatform(localeID);
-    if (lcid > 0)
-    {
-        // Windows found an LCID, return that
-        return lcid;
-    }
-
     uloc_getLanguage(localeID, langID, sizeof(langID), &status);
-    if (U_FAILURE(status)) {
+    if (U_FAILURE(status) || status == U_STRING_NOT_TERMINATED_WARNING) {
         return 0;
     }
 
diff --git a/intl/icu/source/common/wintz.cpp b/intl/icu/source/common/wintz.cpp
index 5e9ac0d2f3..f62cbdb498 100644
--- a/intl/icu/source/common/wintz.cpp
+++ b/intl/icu/source/common/wintz.cpp
@@ -13,7 +13,9 @@
 
 #include "unicode/utypes.h"
 
-#if U_PLATFORM_USES_ONLY_WIN32_API
+// This file contains only desktop Windows behavior
+// Windows UWP calls Windows::Globalization directly, so this isn't needed there.
+#if U_PLATFORM_USES_ONLY_WIN32_API && (U_PLATFORM_HAS_WINUWP_API == 0)
 
 #include "wintz.h"
 #include "cmemory.h"
@@ -21,7 +23,6 @@
 
 #include "unicode/ures.h"
 #include "unicode/ustring.h"
-#include "uresimp.h"
 
 #ifndef WIN32_LEAN_AND_MEAN
 #   define WIN32_LEAN_AND_MEAN
@@ -33,94 +34,362 @@
 #   define NOMCX
 #include <windows.h>
 
-U_NAMESPACE_BEGIN
+#define MAX_LENGTH_ID 40
 
-// The value of MAX_TIMEZONE_ID_LENGTH is 128, which is defined in DYNAMIC_TIME_ZONE_INFORMATION
-#define MAX_TIMEZONE_ID_LENGTH 128
+/* The layout of the Tzi value in the registry */
+typedef struct
+{
+    int32_t bias;
+    int32_t standardBias;
+    int32_t daylightBias;
+    SYSTEMTIME standardDate;
+    SYSTEMTIME daylightDate;
+} TZI;
 
 /**
-* Main Windows time zone detection function.
-* Returns the Windows time zone converted to an ICU time zone as a heap-allocated buffer, or nullptr upon failure.
-* Note: We use the Win32 API GetDynamicTimeZoneInformation to get the current time zone info.
-* This API returns a non-localized time zone name, which we can then map to an ICU time zone name.
+ * Various registry keys and key fragments.
+ */
+static const wchar_t CURRENT_ZONE_REGKEY[] = L"SYSTEM\\CurrentControlSet\\Control\\TimeZoneInformation\\";
+static const char TZI_REGKEY[] = "TZI";
+static const char STD_REGKEY[] = "Std";
+
+/**
+ * The time zone root keys (under HKLM) for Win7+
+ */
+static const char TZ_REGKEY[] = "SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion\\Time Zones\\";
+
+static LONG openTZRegKey(HKEY *hkey, const char *winid)
+{
+    char subKeyName[110]; /* TODO: why 110?? */
+    char *name;
+    LONG result;
+
+    uprv_strcpy(subKeyName, TZ_REGKEY);
+    name = &subKeyName[strlen(subKeyName)];
+    uprv_strcat(subKeyName, winid);
+
+    result = RegOpenKeyExA(HKEY_LOCAL_MACHINE,
+                            subKeyName,
+                            0,
+                            KEY_QUERY_VALUE,
+                            hkey);
+    return result;
+}
+
+static LONG getTZI(const char *winid, TZI *tzi)
+{
+    DWORD cbData = sizeof(TZI);
+    LONG result;
+    HKEY hkey;
+
+    result = openTZRegKey(&hkey, winid);
+
+    if (result == ERROR_SUCCESS)
+    {
+        result = RegQueryValueExA(hkey,
+                                    TZI_REGKEY,
+                                    NULL,
+                                    NULL,
+                                    (LPBYTE)tzi,
+                                    &cbData);
+        RegCloseKey(hkey);
+    }
+
+    return result;
+}
+
+static LONG getSTDName(const char *winid, char *regStdName, int32_t length)
+{
+    DWORD cbData = length;
+    LONG result;
+    HKEY hkey;
+
+    result = openTZRegKey(&hkey, winid);
+
+    if (result == ERROR_SUCCESS)
+    {
+        result = RegQueryValueExA(hkey,
+                                    STD_REGKEY,
+                                    NULL,
+                                    NULL,
+                                    (LPBYTE)regStdName,
+                                    &cbData);
+        RegCloseKey(hkey);
+    }
+
+    return result;
+}
+
+static LONG getTZKeyName(char* tzKeyName, int32_t tzKeyNamelength)
+{
+    HKEY hkey;
+    LONG result = FALSE;
+    WCHAR timeZoneKeyNameData[128];
+    DWORD timeZoneKeyNameLength = static_cast<DWORD>(sizeof(timeZoneKeyNameData));
+
+    if(ERROR_SUCCESS == RegOpenKeyExW(
+        HKEY_LOCAL_MACHINE,
+        CURRENT_ZONE_REGKEY,
+        0,
+        KEY_QUERY_VALUE,
+        &hkey))
+    {
+        if (ERROR_SUCCESS == RegQueryValueExW(
+             hkey,
+             L"TimeZoneKeyName",
+             NULL,
+             NULL,
+             (LPBYTE)timeZoneKeyNameData,
+             &timeZoneKeyNameLength))
+        {
+            // Ensure null termination.
+            timeZoneKeyNameData[UPRV_LENGTHOF(timeZoneKeyNameData) - 1] = L'\0';
+
+            // Convert the UTF-16 string to UTF-8.
+            UErrorCode status = U_ZERO_ERROR;
+            u_strToUTF8(tzKeyName, tzKeyNamelength, NULL, reinterpret_cast<const UChar *>(timeZoneKeyNameData), -1, &status);
+            if (U_ZERO_ERROR == status)
+            {
+                result = ERROR_SUCCESS;
+            }
+        }
+        RegCloseKey(hkey);
+    }
+
+    return result;
+}
+
+/*
+  This code attempts to detect the Windows time zone directly,
+  as set in the Windows Date and Time control panel.  It attempts
+  to work on versions greater than Windows Vista and on localized
+  installs.  It works by directly interrogating the registry and
+  comparing the data there with the data returned by the
+  GetTimeZoneInformation API, along with some other strategies.  The
+  registry contains time zone data under this key:
+
+    HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones\
+
+  Under this key are several subkeys, one for each time zone.  For
+  example these subkeys are named "Pacific Standard Time" on Vista+.
+  There are some other wrinkles; see the code for
+  details.  The subkey name is NOT LOCALIZED, allowing us to support
+  localized installs.
+
+  Under the subkey are data values.  We care about:
+
+    Std   Standard time display name, localized
+    TZI   Binary block of data
+
+  The TZI data is of particular interest.  It contains the offset, two
+  more offsets for standard and daylight time, and the start and end
+  rules.  This is the same data returned by the GetTimeZoneInformation
+  API.  The API may modify the data on the way out, so we have to be
+  careful, but essentially we do a binary comparison against the TZI
+  blocks of various registry keys.  When we find a match, we know what
+  time zone Windows is set to.  Since the registry key is not
+  localized, we can then translate the key through a simple table
+  lookup into the corresponding ICU time zone.
+
+  This strategy doesn't always work because there are zones which
+  share an offset and rules, so more than one TZI block will match.
+  For example, both Tokyo and Seoul are at GMT+9 with no DST rules;
+  their TZI blocks are identical.  For these cases, we fall back to a
+  name lookup.  We attempt to match the display name as stored in the
+  registry for the current zone to the display name stored in the
+  registry for various Windows zones.  By comparing the registry data
+  directly we avoid conversion complications.
+
+  Author: Alan Liu
+  Since: ICU 2.6
+  Based on original code by Carl Brown <cbrown@xnetinc.com>
 */
+
+/**
+ * Main Windows time zone detection function.  Returns the Windows
+ * time zone, translated to an ICU time zone, or NULL upon failure.
+ */
 U_CFUNC const char* U_EXPORT2
 uprv_detectWindowsTimeZone()
 {
     UErrorCode status = U_ZERO_ERROR;
-    char* icuid = nullptr;
-    char dynamicTZKeyName[MAX_TIMEZONE_ID_LENGTH];
-    char tmpid[MAX_TIMEZONE_ID_LENGTH];
+    UResourceBundle* bundle = NULL;
+    char* icuid = NULL;
+    char apiStdName[MAX_LENGTH_ID];
+    char regStdName[MAX_LENGTH_ID];
+    char tmpid[MAX_LENGTH_ID];
     int32_t len;
-    int id = GEOID_NOT_AVAILABLE;
+    int id;
     int errorCode;
-    wchar_t ISOcodeW[3] = {}; /* 2 letter ISO code in UTF-16 */
-    char ISOcode[3] = {}; /* 2 letter ISO code in UTF-8 */
+    wchar_t ISOcodeW[3]; /* 2 letter iso code in UTF-16*/
+    char  ISOcodeA[3]; /* 2 letter iso code in ansi */
 
-    DYNAMIC_TIME_ZONE_INFORMATION dynamicTZI;
-    uprv_memset(&dynamicTZI, 0, sizeof(dynamicTZI));
-    uprv_memset(dynamicTZKeyName, 0, sizeof(dynamicTZKeyName));
-    uprv_memset(tmpid, 0, sizeof(tmpid));
+    LONG result;
+    TZI tziKey;
+    TZI tziReg;
+    TIME_ZONE_INFORMATION apiTZI;
 
-    /* Obtain TIME_ZONE_INFORMATION from the API and get the non-localized time zone name. */
-    if (TIME_ZONE_ID_INVALID == GetDynamicTimeZoneInformation(&dynamicTZI)) {
-        return nullptr;
-    }
+    BOOL tryPreVistaFallback;
+    OSVERSIONINFO osVerInfo;
+
+    /* Obtain TIME_ZONE_INFORMATION from the API, and then convert it
+       to TZI.  We could also interrogate the registry directly; we do
+       this below if needed. */
+    uprv_memset(&apiTZI, 0, sizeof(apiTZI));
+    uprv_memset(&tziKey, 0, sizeof(tziKey));
+    uprv_memset(&tziReg, 0, sizeof(tziReg));
+    GetTimeZoneInformation(&apiTZI);
+    tziKey.bias = apiTZI.Bias;
+    uprv_memcpy((char *)&tziKey.standardDate, (char*)&apiTZI.StandardDate,
+           sizeof(apiTZI.StandardDate));
+    uprv_memcpy((char *)&tziKey.daylightDate, (char*)&apiTZI.DaylightDate,
+           sizeof(apiTZI.DaylightDate));
+
+    /* Convert the wchar_t* standard name to char* */
+    uprv_memset(apiStdName, 0, sizeof(apiStdName));
+    wcstombs(apiStdName, apiTZI.StandardName, MAX_LENGTH_ID);
+
+    tmpid[0] = 0;
 
     id = GetUserGeoID(GEOCLASS_NATION);
     errorCode = GetGeoInfoW(id, GEO_ISO2, ISOcodeW, 3, 0);
+    u_strToUTF8(ISOcodeA, 3, NULL, (const UChar *)ISOcodeW, 3, &status);
 
-    // convert from wchar_t* (UTF-16 on Windows) to char* (UTF-8).
-    u_strToUTF8(ISOcode, UPRV_LENGTHOF(ISOcode), nullptr,
-        reinterpret_cast<const UChar*>(ISOcodeW), UPRV_LENGTHOF(ISOcodeW), &status);
+    bundle = ures_openDirect(NULL, "windowsZones", &status);
+    ures_getByKey(bundle, "mapTimezones", bundle, &status);
 
-    LocalUResourceBundlePointer bundle(ures_openDirect(nullptr, "windowsZones", &status));
-    ures_getByKey(bundle.getAlias(), "mapTimezones", bundle.getAlias(), &status);
-
-    // convert from wchar_t* (UTF-16 on Windows) to char* (UTF-8).
-    u_strToUTF8(dynamicTZKeyName, UPRV_LENGTHOF(dynamicTZKeyName), nullptr,
-        reinterpret_cast<const UChar*>(dynamicTZI.TimeZoneKeyName), UPRV_LENGTHOF(dynamicTZI.TimeZoneKeyName), &status);
-
-    if (U_FAILURE(status)) {
-        return nullptr;
-    }
-
-    if (dynamicTZI.TimeZoneKeyName[0] != 0) {
-        UResourceBundle winTZ;
-        ures_initStackObject(&winTZ);
-        ures_getByKey(bundle.getAlias(), dynamicTZKeyName, &winTZ, &status);
-        
-        if (U_SUCCESS(status)) {
-            const UChar* icuTZ = nullptr;
-            if (errorCode != 0) {
-                icuTZ = ures_getStringByKey(&winTZ, ISOcode, &len, &status);
+    /*
+        Windows Vista+ provides us with a "TimeZoneKeyName" that is not localized
+        and can be used to directly map a name in our bundle. Try to use that first
+        if we're on Vista or higher
+    */
+    uprv_memset(&osVerInfo, 0, sizeof(osVerInfo));
+    osVerInfo.dwOSVersionInfoSize = sizeof(osVerInfo);
+    tryPreVistaFallback = TRUE;
+    result = getTZKeyName(regStdName, sizeof(regStdName));
+    if(ERROR_SUCCESS == result)
+    {
+        UResourceBundle* winTZ = ures_getByKey(bundle, regStdName, NULL, &status);
+        if(U_SUCCESS(status))
+        {
+            const UChar* icuTZ = NULL;
+            if (errorCode != 0)
+            {
+                icuTZ = ures_getStringByKey(winTZ, ISOcodeA, &len, &status);
             }
-            if (errorCode == 0 || icuTZ == nullptr) {
+            if (errorCode==0 || icuTZ==NULL)
+            {
                 /* fallback to default "001" and reset status */
                 status = U_ZERO_ERROR;
-                icuTZ = ures_getStringByKey(&winTZ, "001", &len, &status);
+                icuTZ = ures_getStringByKey(winTZ, "001", &len, &status);
             }
 
-            if (U_SUCCESS(status)) {
-                int index = 0;
-
-                while (!(*icuTZ == '\0' || *icuTZ == ' ')) {
-                    // time zone IDs only contain ASCII invariant characters.
-                    tmpid[index++] = (char)(*icuTZ++);
+            if(U_SUCCESS(status))
+            {
+                int index=0;
+                while (! (*icuTZ == '\0' || *icuTZ ==' '))
+                {
+                    tmpid[index++]=(char)(*icuTZ++);  /* safe to assume 'char' is ASCII compatible on windows */
                 }
-                tmpid[index] = '\0';
+                tmpid[index]='\0';
+                tryPreVistaFallback = FALSE;
             }
         }
-        ures_close(&winTZ);
+        ures_close(winTZ);
     }
 
-    // Copy the timezone ID to icuid to be returned.
-    if (tmpid[0] != 0) {
-        icuid = uprv_strdup(tmpid);
+    if(tryPreVistaFallback)
+    {
+        /* Note: We get the winid not from static tables but from resource bundle. */
+        while (U_SUCCESS(status) && ures_hasNext(bundle))
+        {
+            UBool idFound = FALSE;
+            const char* winid;
+            UResourceBundle* winTZ = ures_getNextResource(bundle, NULL, &status);
+            if (U_FAILURE(status))
+            {
+                break;
+            }
+            winid = ures_getKey(winTZ);
+            result = getTZI(winid, &tziReg);
+
+            if (result == ERROR_SUCCESS)
+            {
+                /* Windows alters the DaylightBias in some situations.
+                   Using the bias and the rules suffices, so overwrite
+                   these unreliable fields. */
+                tziKey.standardBias = tziReg.standardBias;
+                tziKey.daylightBias = tziReg.daylightBias;
+
+                if (uprv_memcmp((char *)&tziKey, (char*)&tziReg, sizeof(tziKey)) == 0)
+                {
+                    const UChar* icuTZ = NULL;
+                    if (errorCode != 0)
+                    {
+                        icuTZ = ures_getStringByKey(winTZ, ISOcodeA, &len, &status);
+                    }
+                    if (errorCode==0 || icuTZ==NULL)
+                    {
+                        /* fallback to default "001" and reset status */
+                        status = U_ZERO_ERROR;
+                        icuTZ = ures_getStringByKey(winTZ, "001", &len, &status);
+                    }
+
+                    if (U_SUCCESS(status))
+                    {
+                        /* Get the standard name from the registry key to compare with
+                           the one from Windows API call. */
+                        uprv_memset(regStdName, 0, sizeof(regStdName));
+                        result = getSTDName(winid, regStdName, sizeof(regStdName));
+                        if (result == ERROR_SUCCESS)
+                        {
+                            if (uprv_strcmp(apiStdName, regStdName) == 0)
+                            {
+                                idFound = TRUE;
+                            }
+                        }
+
+                        /* tmpid buffer holds the ICU timezone ID corresponding to the timezone ID from Windows.
+                         * If none is found, tmpid buffer will contain a fallback ID (i.e. the time zone ID matching
+                         * the current time zone information)
+                         */
+                        if (idFound || tmpid[0] == 0)
+                        {
+                            /* if icuTZ has more than one city, take only the first (i.e. terminate icuTZ at first space) */
+                            int index=0;
+                            while (! (*icuTZ == '\0' || *icuTZ ==' '))
+                            {
+                                tmpid[index++]=(char)(*icuTZ++);  /* safe to assume 'char' is ASCII compatible on windows */
+                            }
+                            tmpid[index]='\0';
+                        }
+                    }
+                }
+            }
+            ures_close(winTZ);
+            if (idFound)
+            {
+                break;
+            }
+        }
     }
 
+    /*
+     * Copy the timezone ID to icuid to be returned.
+     */
+    if (tmpid[0] != 0)
+    {
+        len = uprv_strlen(tmpid);
+        icuid = (char*)uprv_calloc(len + 1, sizeof(char));
+        if (icuid != NULL)
+        {
+            uprv_strcpy(icuid, tmpid);
+        }
+    }
+
+    ures_close(bundle);
+
     return icuid;
 }
 
-U_NAMESPACE_END
-#endif /* U_PLATFORM_USES_ONLY_WIN32_API  */
+#endif /* U_PLATFORM_USES_ONLY_WIN32_API && (U_PLATFORM_HAS_WINUWP_API == 0) */
diff --git a/intl/icu/source/common/wintz.h b/intl/icu/source/common/wintz.h
index f98b1779b5..cd8565eef1 100644
--- a/intl/icu/source/common/wintz.h
+++ b/intl/icu/source/common/wintz.h
@@ -28,7 +28,7 @@ U_CDECL_BEGIN
 typedef struct _TIME_ZONE_INFORMATION TIME_ZONE_INFORMATION;
 U_CDECL_END
 
-U_CFUNC const char* U_EXPORT2
+U_INTERNAL const char* U_EXPORT2
 uprv_detectWindowsTimeZone();
 
 #endif /* U_PLATFORM_USES_ONLY_WIN32_API  */
diff --git a/intl/icu/source/i18n/windtfmt.cpp b/intl/icu/source/i18n/windtfmt.cpp
index 6a3779a29e..043469bd7d 100644
--- a/intl/icu/source/i18n/windtfmt.cpp
+++ b/intl/icu/source/i18n/windtfmt.cpp
@@ -94,83 +94,12 @@ UnicodeString* Win32DateFormat::getTimeDateFormat(const Calendar *cal, const Loc
     return result;
 }
 
-// TODO: This is copied in both winnmfmt.cpp and windtfmt.cpp, but really should
-// be factored out into a common helper for both.
-static UErrorCode GetEquivalentWindowsLocaleName(const Locale& locale, UnicodeString** buffer)
-{
-    UErrorCode status = U_ZERO_ERROR;
-    char asciiBCP47Tag[LOCALE_NAME_MAX_LENGTH] = {};
-
-    // Convert from names like "en_CA" and "de_DE@collation=phonebook" to "en-CA" and "de-DE-u-co-phonebk".
-    (void)uloc_toLanguageTag(locale.getName(), asciiBCP47Tag, UPRV_LENGTHOF(asciiBCP47Tag), FALSE, &status);
-
-    if (U_SUCCESS(status))
-    {
-        // Need it to be UTF-16, not 8-bit
-        // TODO: This seems like a good thing for a helper
-        wchar_t bcp47Tag[LOCALE_NAME_MAX_LENGTH] = {};
-        int32_t i;
-        for (i = 0; i < UPRV_LENGTHOF(bcp47Tag); i++)
-        {
-            if (asciiBCP47Tag[i] == '\0')
-            {
-                break;
-            }
-            else
-            {
-                // normally just copy the character
-                bcp47Tag[i] = static_cast<wchar_t>(asciiBCP47Tag[i]);
-            }
-        }
-
-        // Ensure it's null terminated
-        if (i < (UPRV_LENGTHOF(bcp47Tag) - 1))
-        {
-            bcp47Tag[i] = L'\0';
-        }
-        else
-        {
-            // Ran out of room.
-            bcp47Tag[UPRV_LENGTHOF(bcp47Tag) - 1] = L'\0';
-        }
-
-
-        wchar_t windowsLocaleName[LOCALE_NAME_MAX_LENGTH] = {};
-
-        // Note: On Windows versions below 10, there is no support for locale name aliases.
-        // This means that it will fail for locales where ICU has a completely different
-        // name (like ku vs ckb), and it will also not work for alternate sort locale
-        // names like "de-DE-u-co-phonebk".
-        
-        // TODO: We could add some sort of exception table for cases like ku vs ckb.
-
-        int length = ResolveLocaleName(bcp47Tag, windowsLocaleName, UPRV_LENGTHOF(windowsLocaleName));
-
-        if (length > 0)
-        {
-            *buffer = new UnicodeString(windowsLocaleName);
-        }
-        else
-        {
-            status = U_UNSUPPORTED_ERROR;
-        }
-    }
-    return status;
-}
-
 // TODO: Range-check timeStyle, dateStyle
 Win32DateFormat::Win32DateFormat(DateFormat::EStyle timeStyle, DateFormat::EStyle dateStyle, const Locale &locale, UErrorCode &status)
-  : DateFormat(), fDateTimeMsg(NULL), fTimeStyle(timeStyle), fDateStyle(dateStyle), fLocale(locale), fZoneID(), fWindowsLocaleName(nullptr)
+  : DateFormat(), fDateTimeMsg(NULL), fTimeStyle(timeStyle), fDateStyle(dateStyle), fLocale(locale), fZoneID()
 {
     if (U_SUCCESS(status)) {
-        GetEquivalentWindowsLocaleName(locale, &fWindowsLocaleName);
-        // Note: In the previous code, it would look up the LCID for the locale, and if
-        // the locale was not recognized then it would get an LCID of 0, which is a
-        // synonym for LOCALE_USER_DEFAULT on Windows.
-        // If the above method fails, then fWindowsLocaleName will remain as nullptr, and 
-        // then we will pass nullptr to API GetLocaleInfoEx, which is the same as passing
-        // LOCALE_USER_DEFAULT.
-
+        fLCID = locale.getLCID();
         fTZI = NEW_ARRAY(TIME_ZONE_INFORMATION, 1);
         uprv_memset(fTZI, 0, sizeof(TIME_ZONE_INFORMATION));
         adoptCalendar(Calendar::createInstance(locale, status));
@@ -188,7 +117,6 @@ Win32DateFormat::~Win32DateFormat()
 //    delete fCalendar;
     uprv_free(fTZI);
     delete fDateTimeMsg;
-    delete fWindowsLocaleName;
 }
 
 Win32DateFormat &Win32DateFormat::operator=(const Win32DateFormat &other)
@@ -208,8 +136,6 @@ Win32DateFormat &Win32DateFormat::operator=(const Win32DateFormat &other)
     this->fTZI = NEW_ARRAY(TIME_ZONE_INFORMATION, 1);
     *this->fTZI = *other.fTZI;
 
-    this->fWindowsLocaleName = other.fWindowsLocaleName == NULL ? NULL : new UnicodeString(*other.fWindowsLocaleName);
-
     return *this;
 }
 
@@ -309,22 +235,16 @@ void Win32DateFormat::formatDate(const SYSTEMTIME *st, UnicodeString &appendTo)
     int result=0;
     wchar_t stackBuffer[STACK_BUFFER_SIZE];
     wchar_t *buffer = stackBuffer;
-    const wchar_t *localeName = nullptr;
 
-    if (fWindowsLocaleName != nullptr)
-    {
-        localeName = reinterpret_cast<const wchar_t*>(toOldUCharPtr(fWindowsLocaleName->getTerminatedBuffer()));
-    }
-
-    result = GetDateFormatEx(localeName, dfFlags[fDateStyle - kDateOffset], st, NULL, buffer, STACK_BUFFER_SIZE, NULL);
+    result = GetDateFormatW(fLCID, dfFlags[fDateStyle - kDateOffset], st, NULL, buffer, STACK_BUFFER_SIZE);
 
     if (result == 0) {
         if (GetLastError() == ERROR_INSUFFICIENT_BUFFER) {
-            int newLength = GetDateFormatEx(localeName, dfFlags[fDateStyle - kDateOffset], st, NULL, NULL, 0, NULL);
+            int newLength = GetDateFormatW(fLCID, dfFlags[fDateStyle - kDateOffset], st, NULL, NULL, 0);
 
             buffer = NEW_ARRAY(wchar_t, newLength);
 
-            GetDateFormatEx(localeName, dfFlags[fDateStyle - kDateOffset], st, NULL, buffer, newLength, NULL);
+            GetDateFormatW(fLCID, dfFlags[fDateStyle - kDateOffset], st, NULL, buffer, newLength);
         }
     }
 
@@ -342,22 +262,16 @@ void Win32DateFormat::formatTime(const SYSTEMTIME *st, UnicodeString &appendTo)
     int result;
     wchar_t stackBuffer[STACK_BUFFER_SIZE];
     wchar_t *buffer = stackBuffer;
-    const wchar_t *localeName = nullptr;
 
-    if (fWindowsLocaleName != nullptr)
-    {
-        localeName = reinterpret_cast<const wchar_t*>(toOldUCharPtr(fWindowsLocaleName->getTerminatedBuffer()));
-    }
-
-    result = GetTimeFormatEx(localeName, tfFlags[fTimeStyle], st, NULL, buffer, STACK_BUFFER_SIZE);
+    result = GetTimeFormatW(fLCID, tfFlags[fTimeStyle], st, NULL, buffer, STACK_BUFFER_SIZE);
 
     if (result == 0) {
         if (GetLastError() == ERROR_INSUFFICIENT_BUFFER) {
-            int newLength = GetTimeFormatEx(localeName, tfFlags[fTimeStyle], st, NULL, NULL, 0);
+            int newLength = GetTimeFormatW(fLCID, tfFlags[fTimeStyle], st, NULL, NULL, 0);
 
             buffer = NEW_ARRAY(wchar_t, newLength);
 
-            GetTimeFormatEx(localeName, tfFlags[fTimeStyle], st, NULL, buffer, newLength);
+            GetDateFormatW(fLCID, tfFlags[fTimeStyle], st, NULL, buffer, newLength);
         }
     }
 
diff --git a/intl/icu/source/i18n/windtfmt.h b/intl/icu/source/i18n/windtfmt.h
index 33069058fb..9d457c18ae 100644
--- a/intl/icu/source/i18n/windtfmt.h
+++ b/intl/icu/source/i18n/windtfmt.h
@@ -124,10 +124,9 @@ private:
     DateFormat::EStyle fTimeStyle;
     DateFormat::EStyle fDateStyle;
     Locale fLocale;
+    int32_t fLCID;
     UnicodeString fZoneID;
     TIME_ZONE_INFORMATION *fTZI;
-
-    UnicodeString* fWindowsLocaleName; // Stores the equivalent Windows locale name.
 };
 
 U_NAMESPACE_END
diff --git a/intl/icu/source/i18n/winnmfmt.cpp b/intl/icu/source/i18n/winnmfmt.cpp
index 5637a0f4a1..9660fa9ea5 100644
--- a/intl/icu/source/i18n/winnmfmt.cpp
+++ b/intl/icu/source/i18n/winnmfmt.cpp
@@ -60,43 +60,43 @@ UOBJECT_DEFINE_RTTI_IMPLEMENTATION(Win32NumberFormat)
  * end in ";0" then the return value should be multiplied by 10.
  * (e.g. "3" => 30, "3;2" => 320)
  */
-static UINT getGrouping(const wchar_t *grouping)
+static UINT getGrouping(const char *grouping)
 {
     UINT g = 0;
-    const wchar_t *s;
+	const char *s;
 
-    for (s = grouping; *s != L'\0'; s += 1) {
-        if (*s > L'0' && *s < L'9') {
-            g = g * 10 + (*s - L'0');
-        } else if (*s != L';') {
+    for (s = grouping; *s != '\0'; s += 1) {
+        if (*s > '0' && *s < '9') {
+            g = g * 10 + (*s - '0');
+        } else if (*s != ';') {
             break;
         }
     }
 
-    if (*s != L'0') {
+    if (*s != '0') {
         g *= 10;
     }
 
     return g;
 }
 
-static void getNumberFormat(NUMBERFMTW *fmt, const wchar_t *windowsLocaleName)
+static void getNumberFormat(NUMBERFMTW *fmt, int32_t lcid)
 {
-    wchar_t buf[10];
+    char buf[10];
 
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_RETURN_NUMBER|LOCALE_IDIGITS, (LPWSTR) &fmt->NumDigits, sizeof(UINT));
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_RETURN_NUMBER|LOCALE_ILZERO,  (LPWSTR) &fmt->LeadingZero, sizeof(UINT));
+    GetLocaleInfoW(lcid, LOCALE_RETURN_NUMBER|LOCALE_IDIGITS, (LPWSTR) &fmt->NumDigits, sizeof(UINT));
+    GetLocaleInfoW(lcid, LOCALE_RETURN_NUMBER|LOCALE_ILZERO,  (LPWSTR) &fmt->LeadingZero, sizeof(UINT));
 
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_SGROUPING, (LPWSTR)buf, 10);
+    GetLocaleInfoA(lcid, LOCALE_SGROUPING, buf, 10);
     fmt->Grouping = getGrouping(buf);
 
     fmt->lpDecimalSep = NEW_ARRAY(wchar_t, 6);
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_SDECIMAL,  fmt->lpDecimalSep,  6);
+    GetLocaleInfoW(lcid, LOCALE_SDECIMAL,  fmt->lpDecimalSep,  6);
 
     fmt->lpThousandSep = NEW_ARRAY(wchar_t, 6);
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_STHOUSAND, fmt->lpThousandSep, 6);
+    GetLocaleInfoW(lcid, LOCALE_STHOUSAND, fmt->lpThousandSep, 6);
 
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_RETURN_NUMBER|LOCALE_INEGNUMBER, (LPWSTR) &fmt->NegativeOrder, sizeof(UINT));
+    GetLocaleInfoW(lcid, LOCALE_RETURN_NUMBER|LOCALE_INEGNUMBER, (LPWSTR) &fmt->NegativeOrder, sizeof(UINT));
 }
 
 static void freeNumberFormat(NUMBERFMTW *fmt)
@@ -107,27 +107,27 @@ static void freeNumberFormat(NUMBERFMTW *fmt)
     }
 }
 
-static void getCurrencyFormat(CURRENCYFMTW *fmt, const wchar_t *windowsLocaleName)
+static void getCurrencyFormat(CURRENCYFMTW *fmt, int32_t lcid)
 {
-    wchar_t buf[10];
+    char buf[10];
 
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_RETURN_NUMBER|LOCALE_ICURRDIGITS, (LPWSTR) &fmt->NumDigits, sizeof(UINT));
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_RETURN_NUMBER|LOCALE_ILZERO, (LPWSTR) &fmt->LeadingZero, sizeof(UINT));
+    GetLocaleInfoW(lcid, LOCALE_RETURN_NUMBER|LOCALE_ICURRDIGITS, (LPWSTR) &fmt->NumDigits, sizeof(UINT));
+    GetLocaleInfoW(lcid, LOCALE_RETURN_NUMBER|LOCALE_ILZERO, (LPWSTR) &fmt->LeadingZero, sizeof(UINT));
 
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_SMONGROUPING, (LPWSTR)buf, sizeof(buf));
+    GetLocaleInfoA(lcid, LOCALE_SMONGROUPING, buf, sizeof(buf));
     fmt->Grouping = getGrouping(buf);
 
     fmt->lpDecimalSep = NEW_ARRAY(wchar_t, 6);
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_SMONDECIMALSEP,  fmt->lpDecimalSep,  6);
+    GetLocaleInfoW(lcid, LOCALE_SMONDECIMALSEP,  fmt->lpDecimalSep,  6);
 
     fmt->lpThousandSep = NEW_ARRAY(wchar_t, 6);
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_SMONTHOUSANDSEP, fmt->lpThousandSep, 6);
+    GetLocaleInfoW(lcid, LOCALE_SMONTHOUSANDSEP, fmt->lpThousandSep, 6);
 
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_RETURN_NUMBER|LOCALE_INEGCURR,  (LPWSTR) &fmt->NegativeOrder, sizeof(UINT));
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_RETURN_NUMBER|LOCALE_ICURRENCY, (LPWSTR) &fmt->PositiveOrder, sizeof(UINT));
+    GetLocaleInfoW(lcid, LOCALE_RETURN_NUMBER|LOCALE_INEGCURR,  (LPWSTR) &fmt->NegativeOrder, sizeof(UINT));
+    GetLocaleInfoW(lcid, LOCALE_RETURN_NUMBER|LOCALE_ICURRENCY, (LPWSTR) &fmt->PositiveOrder, sizeof(UINT));
 
     fmt->lpCurrencySymbol = NEW_ARRAY(wchar_t, 8);
-    GetLocaleInfoEx(windowsLocaleName, LOCALE_SCURRENCY, (LPWSTR) fmt->lpCurrencySymbol, 8);
+    GetLocaleInfoW(lcid, LOCALE_SCURRENCY, (LPWSTR) fmt->lpCurrencySymbol, 8);
 }
 
 static void freeCurrencyFormat(CURRENCYFMTW *fmt)
@@ -139,83 +139,12 @@ static void freeCurrencyFormat(CURRENCYFMTW *fmt)
     }
 }
 
-// TODO: This is copied in both winnmfmt.cpp and windtfmt.cpp, but really should
-// be factored out into a common helper for both.
-static UErrorCode GetEquivalentWindowsLocaleName(const Locale& locale, UnicodeString** buffer)
-{
-    UErrorCode status = U_ZERO_ERROR;
-    char asciiBCP47Tag[LOCALE_NAME_MAX_LENGTH] = {};
-
-    // Convert from names like "en_CA" and "de_DE@collation=phonebook" to "en-CA" and "de-DE-u-co-phonebk".
-    (void) uloc_toLanguageTag(locale.getName(), asciiBCP47Tag, UPRV_LENGTHOF(asciiBCP47Tag), FALSE, &status);
-
-    if (U_SUCCESS(status))
-    {
-        // Need it to be UTF-16, not 8-bit
-        // TODO: This seems like a good thing for a helper
-        wchar_t bcp47Tag[LOCALE_NAME_MAX_LENGTH] = {};
-        int32_t i;
-        for (i = 0; i < UPRV_LENGTHOF(bcp47Tag); i++)
-        {
-            if (asciiBCP47Tag[i] == '\0')
-            {
-                break;
-            }
-            else
-            {
-                // normally just copy the character
-                bcp47Tag[i] = static_cast<wchar_t>(asciiBCP47Tag[i]);
-            }
-        }
-
-        // Ensure it's null terminated
-        if (i < (UPRV_LENGTHOF(bcp47Tag) - 1))
-        {
-            bcp47Tag[i] = L'\0';
-        }
-        else
-        {
-            // Ran out of room.
-            bcp47Tag[UPRV_LENGTHOF(bcp47Tag) - 1] = L'\0';
-        }
-
-
-        wchar_t windowsLocaleName[LOCALE_NAME_MAX_LENGTH] = {};
-
-        // Note: On Windows versions below 10, there is no support for locale name aliases.
-        // This means that it will fail for locales where ICU has a completely different
-        // name (like ku vs ckb), and it will also not work for alternate sort locale
-        // names like "de-DE-u-co-phonebk".
-
-        // TODO: We could add some sort of exception table for cases like ku vs ckb.
-
-        int length = ResolveLocaleName(bcp47Tag, windowsLocaleName, UPRV_LENGTHOF(windowsLocaleName));
-
-        if (length > 0)
-        {
-            *buffer = new UnicodeString(windowsLocaleName);
-        }
-        else
-        {
-            status = U_UNSUPPORTED_ERROR;
-        }
-    }
-    return status;
-}
-
 Win32NumberFormat::Win32NumberFormat(const Locale &locale, UBool currency, UErrorCode &status)
-  : NumberFormat(), fCurrency(currency), fFormatInfo(NULL), fFractionDigitsSet(FALSE), fWindowsLocaleName(nullptr)
+  : NumberFormat(), fCurrency(currency), fFormatInfo(NULL), fFractionDigitsSet(FALSE)
 {
     if (!U_FAILURE(status)) {
         fLCID = locale.getLCID();
 
-        GetEquivalentWindowsLocaleName(locale, &fWindowsLocaleName);
-        // Note: In the previous code, it would look up the LCID for the locale, and if
-        // the locale was not recognized then it would get an LCID of 0, which is a
-        // synonym for LOCALE_USER_DEFAULT on Windows.
-        // If the above method fails, then fWindowsLocaleName will remain as nullptr, and 
-        // then we will pass nullptr to API GetLocaleInfoEx, which is the same as passing
-        // LOCALE_USER_DEFAULT.
 
         // Resolve actual locale to be used later
         UErrorCode tmpsts = U_ZERO_ERROR;
@@ -226,19 +155,12 @@ Win32NumberFormat::Win32NumberFormat(const Locale &locale, UBool currency, UErro
             fLocale = Locale((const char*)tmpLocID);
         }
 
-        const wchar_t *localeName = nullptr;
-
-        if (fWindowsLocaleName != nullptr)
-        {
-            localeName = reinterpret_cast<const wchar_t*>(toOldUCharPtr(fWindowsLocaleName->getTerminatedBuffer()));
-        }
-
         fFormatInfo = (FormatInfo*)uprv_malloc(sizeof(FormatInfo));
 
         if (fCurrency) {
-            getCurrencyFormat(&fFormatInfo->currency, localeName);
+            getCurrencyFormat(&fFormatInfo->currency, fLCID);
         } else {
-            getNumberFormat(&fFormatInfo->number, localeName);
+            getNumberFormat(&fFormatInfo->number, fLCID);
         }
     }
 }
@@ -263,7 +185,6 @@ Win32NumberFormat::~Win32NumberFormat()
 
         uprv_free(fFormatInfo);
     }
-    delete fWindowsLocaleName;
 }
 
 Win32NumberFormat &Win32NumberFormat::operator=(const Win32NumberFormat &other)
@@ -274,21 +195,13 @@ Win32NumberFormat &Win32NumberFormat::operator=(const Win32NumberFormat &other)
     this->fLocale            = other.fLocale;
     this->fLCID              = other.fLCID;
     this->fFractionDigitsSet = other.fFractionDigitsSet;
-    this->fWindowsLocaleName = other.fWindowsLocaleName == NULL ? NULL : new UnicodeString(*other.fWindowsLocaleName);
     
-    const wchar_t *localeName = nullptr;
-
-    if (fWindowsLocaleName != nullptr)
-    {
-        localeName = reinterpret_cast<const wchar_t*>(toOldUCharPtr(fWindowsLocaleName->getTerminatedBuffer()));
-    }
-
     if (fCurrency) {
         freeCurrencyFormat(&fFormatInfo->currency);
-        getCurrencyFormat(&fFormatInfo->currency, localeName);
+        getCurrencyFormat(&fFormatInfo->currency, fLCID);
     } else {
         freeNumberFormat(&fFormatInfo->number);
-        getNumberFormat(&fFormatInfo->number, localeName);
+        getNumberFormat(&fFormatInfo->number, fLCID);
     }
 
     return *this;
@@ -389,13 +302,6 @@ UnicodeString &Win32NumberFormat::format(int32_t numDigits, UnicodeString &appen
     formatInfo = *fFormatInfo;
     buffer[0] = 0x0000;
 
-    const wchar_t *localeName = nullptr;
-
-    if (fWindowsLocaleName != nullptr)
-    {
-        localeName = reinterpret_cast<const wchar_t*>(toOldUCharPtr(fWindowsLocaleName->getTerminatedBuffer()));
-    }
-
     if (fCurrency) {
         if (fFractionDigitsSet) {
             formatInfo.currency.NumDigits = (UINT) numDigits;
@@ -405,17 +311,17 @@ UnicodeString &Win32NumberFormat::format(int32_t numDigits, UnicodeString &appen
             formatInfo.currency.Grouping = 0;
         }
 
-        result = GetCurrencyFormatEx(localeName, 0, nBuffer, &formatInfo.currency, buffer, STACK_BUFFER_SIZE);
+        result = GetCurrencyFormatW(fLCID, 0, nBuffer, &formatInfo.currency, buffer, STACK_BUFFER_SIZE);
 
         if (result == 0) {
             DWORD lastError = GetLastError();
 
             if (lastError == ERROR_INSUFFICIENT_BUFFER) {
-                int newLength = GetCurrencyFormatEx(localeName, 0, nBuffer, &formatInfo.currency, NULL, 0);
+                int newLength = GetCurrencyFormatW(fLCID, 0, nBuffer, &formatInfo.currency, NULL, 0);
 
                 buffer = NEW_ARRAY(wchar_t, newLength);
                 buffer[0] = 0x0000;
-                GetCurrencyFormatEx(localeName, 0, nBuffer,  &formatInfo.currency, buffer, newLength);
+                GetCurrencyFormatW(fLCID, 0, nBuffer,  &formatInfo.currency, buffer, newLength);
             }
         }
     } else {
@@ -427,15 +333,15 @@ UnicodeString &Win32NumberFormat::format(int32_t numDigits, UnicodeString &appen
             formatInfo.number.Grouping = 0;
         }
 
-        result = GetNumberFormatEx(localeName, 0, nBuffer, &formatInfo.number, buffer, STACK_BUFFER_SIZE);
+        result = GetNumberFormatW(fLCID, 0, nBuffer, &formatInfo.number, buffer, STACK_BUFFER_SIZE);
 
         if (result == 0) {
             if (GetLastError() == ERROR_INSUFFICIENT_BUFFER) {
-                int newLength = GetNumberFormatEx(localeName, 0, nBuffer, &formatInfo.number, NULL, 0);
+                int newLength = GetNumberFormatW(fLCID, 0, nBuffer, &formatInfo.number, NULL, 0);
 
                 buffer = NEW_ARRAY(wchar_t, newLength);
                 buffer[0] = 0x0000;
-                GetNumberFormatEx(localeName, 0, nBuffer, &formatInfo.number, buffer, newLength);
+                GetNumberFormatW(fLCID, 0, nBuffer, &formatInfo.number, buffer, newLength);
             }
         }
     }
diff --git a/intl/icu/source/i18n/winnmfmt.h b/intl/icu/source/i18n/winnmfmt.h
index 6139c1ff68..331d535495 100644
--- a/intl/icu/source/i18n/winnmfmt.h
+++ b/intl/icu/source/i18n/winnmfmt.h
@@ -154,8 +154,6 @@ private:
     int32_t fLCID;
     FormatInfo *fFormatInfo;
     UBool fFractionDigitsSet;
-
-    UnicodeString* fWindowsLocaleName; // Stores the equivalent Windows locale name.
 };
 
 U_NAMESPACE_END