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// © 2016 and later: Unicode, Inc. and others.
// License & terms of use:
* Copyright (C) 2005-2015, International Business Machines
* Corporation and others. All Rights Reserved.
#include "unicode/utypes.h"
#include "wintz.h"
#include "charstr.h"
#include "cmemory.h"
#include "cstring.h"
#include "unicode/ures.h"
#include "unicode/unistr.h"
#include "uresimp.h"
# define WIN32_LEAN_AND_MEAN
# define NOUSER
# define NOSERVICE
# define NOIME
# define NOMCX
#include <windows.h>
// Note these constants and the struct are only used when dealing with the fallback path for RDP sesssions.
// This is the location of the time zones in the registry on Vista+ systems.
// See:
#define WINDOWS_TIMEZONES_REG_KEY_PATH L"SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion\\Time Zones"
// Max length for a registry key is 255. +1 for null.
// See:
// This is the layout of the TZI binary value in the registry.
// See:
typedef struct _REG_TZI_FORMAT {
LONG Bias;
LONG StandardBias;
LONG DaylightBias;
SYSTEMTIME StandardDate;
SYSTEMTIME DaylightDate;
* This is main Windows time zone detection function.
* It returns the Windows time zone converted to an ICU time zone as a heap-allocated buffer, or nullptr upon failure.
* We use the Win32 API GetDynamicTimeZoneInformation (which is available since Vista) to get the current time zone info,
* as this API returns a non-localized time zone name which can be then mapped to an ICU time zone.
* However, in some RDP/terminal services situations, this struct isn't always fully complete, and the TimeZoneKeyName
* field of the struct might be NULL. This can happen with some 3rd party RDP clients, and also when using older versions
* of the RDP protocol, which don't send the newer TimeZoneKeyNamei information and only send the StandardName and DaylightName.
* Since these 3rd party clients and older RDP clients only send the pre-Vista time zone information to the server, this means that we
* need to fallback on using the pre-Vista methods to determine the time zone. This unfortunately requires examining the registry directly
* in order to try and determine the current time zone.
* Note that this can however still fail in some cases though if the client and server are using different languages, as the StandardName
* that is sent by client is localized in the client's language. However, we must compare this to the names that are on the server, which
* are localized in registry using the server's language. Despite that, this is the best we can do.
* Note: This fallback method won't work for the UWP version though, as we can't use the registry APIs in UWP.
* Once we have the current Windows time zone, then we can then map it to an ICU time zone ID (~ Olsen ID).
U_CAPI const char* U_EXPORT2
// We first try to obtain the time zone directly by using the TimeZoneKeyName field of the DYNAMIC_TIME_ZONE_INFORMATION struct.
uprv_memset(&dynamicTZI, 0, sizeof(dynamicTZI));
SYSTEMTIME systemTimeAllZero;
uprv_memset(&systemTimeAllZero, 0, sizeof(systemTimeAllZero));
if (GetDynamicTimeZoneInformation(&dynamicTZI) == TIME_ZONE_ID_INVALID) {
return nullptr;
// If the DST setting has been turned off in the Control Panel, then return "Etc/GMT<offset>".
// Note: This logic is based on how the Control Panel itself determines if DST is 'off' on Windows.
// The code is somewhat convoluted; in a sort of pseudo-code it looks like this:
// IF (GetDynamicTimeZoneInformation != TIME_ZONE_ID_INVALID) && (DynamicDaylightTimeDisabled != 0) &&
// (StandardDate == DaylightDate) &&
// (
// (TimeZoneKeyName != Empty && StandardDate == 0) ||
// (TimeZoneKeyName == Empty && StandardDate != 0)
// )
// DST setting is "Disabled".
if (dynamicTZI.DynamicDaylightTimeDisabled != 0 &&
uprv_memcmp(&dynamicTZI.StandardDate, &dynamicTZI.DaylightDate, sizeof(dynamicTZI.StandardDate)) == 0 &&
((dynamicTZI.TimeZoneKeyName[0] != L'\0' && uprv_memcmp(&dynamicTZI.StandardDate, &systemTimeAllZero, sizeof(systemTimeAllZero)) == 0) ||
(dynamicTZI.TimeZoneKeyName[0] == L'\0' && uprv_memcmp(&dynamicTZI.StandardDate, &systemTimeAllZero, sizeof(systemTimeAllZero)) != 0)))
LONG utcOffsetMins = dynamicTZI.Bias;
if (utcOffsetMins == 0) {
return uprv_strdup("Etc/UTC");
// No way to support when DST is turned off and the offset in minutes is not a multiple of 60.
if (utcOffsetMins % 60 == 0) {
char gmtOffsetTz[11] = {}; // "Etc/GMT+dd" is 11-char long with a terminal null.
// Important note on the sign convention for zones:
// From
// "In order to conform with the POSIX style, those zone names beginning with "Etc/GMT" have their sign reversed
// from the standard ISO 8601 convention. In the "Etc" area, zones west of GMT have a positive sign and those
// east have a negative sign in their name (e.g "Etc/GMT-14" is 14 hours ahead of GMT)."
// Regarding the POSIX style, from
// "The offset specifies the time value you must add to the local time to get a Coordinated Universal Time value."
// However, the Bias value in DYNAMIC_TIME_ZONE_INFORMATION *already* follows the POSIX convention.
// From
// "The bias is the difference, in minutes, between Coordinated Universal Time (UTC) and
// local time. All translations between UTC and local time are based on the following formula:
// UTC = local time + bias"
// For example, a time zone that is 3 hours ahead of UTC (UTC+03:00) would have a Bias value of -180, and the
// corresponding time zone ID would be "Etc/GMT-3". (So there is no need to negate utcOffsetMins below.)
int ret = snprintf(gmtOffsetTz, UPRV_LENGTHOF(gmtOffsetTz), "Etc/GMT%+ld", utcOffsetMins / 60);
if (ret > 0 && ret < UPRV_LENGTHOF(gmtOffsetTz)) {
return uprv_strdup(gmtOffsetTz);
// If DST is NOT disabled, but the TimeZoneKeyName field of the struct is NULL, then we may be dealing with a
// RDP/terminal services session where the 'Time Zone Redirection' feature is enabled. However, either the RDP
// client sent the server incomplete info (some 3rd party RDP clients only send the StandardName and DaylightName,
// but do not send the important TimeZoneKeyName), or if the RDP server has not appropriately populated the struct correctly.
// In this case we unfortunately have no choice but to fallback to using the pre-Vista method of determining the
// time zone, which requires examining the registry directly.
// Note that this can however still fail though if the client and server are using different languages, as the StandardName
// that is sent by client is *localized* in the client's language. However, we must compare this to the names that are
// on the server, which are *localized* in registry using the server's language.
// One other note is that this fallback method doesn't work for the UWP version, as we can't use the registry APIs.
// windowsTimeZoneName will point at timezoneSubKeyName if we had to fallback to using the registry, and we found a match.
WCHAR *windowsTimeZoneName = dynamicTZI.TimeZoneKeyName;
if (dynamicTZI.TimeZoneKeyName[0] == 0) {
// We can't use the registry APIs in the UWP version.
(void)timezoneSubKeyName; // suppress unused variable warnings.
return nullptr;
// Open the path to the time zones in the Windows registry.
LONG ret;
HKEY hKeyAllTimeZones = nullptr;
if (ret != ERROR_SUCCESS) {
// If we can't open the key, then we can't do much, so fail.
return nullptr;
// Read the number of subkeys under the time zone registry path.
DWORD numTimeZoneSubKeys;
ret = RegQueryInfoKeyW(hKeyAllTimeZones, nullptr, nullptr, nullptr, &numTimeZoneSubKeys,
nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr);
if (ret != ERROR_SUCCESS) {
return nullptr;
// Examine each of the subkeys to try and find a match for the localized standard name ("Std").
// Note: The name of the time zone subkey itself is not localized, but the "Std" name is localized. This means
// that we could fail to find a match if the RDP client and RDP server are using different languages, but unfortunately
// there isn't much we can do about it.
HKEY hKeyTimeZoneSubKey = nullptr;
ULONG registryValueType;
for (DWORD i = 0; i < numTimeZoneSubKeys; i++) {
// Note: RegEnumKeyExW wants the size of the buffer in characters.
DWORD size = UPRV_LENGTHOF(timezoneSubKeyName);
ret = RegEnumKeyExW(hKeyAllTimeZones, i, timezoneSubKeyName, &size, nullptr, nullptr, nullptr, nullptr);
if (ret != ERROR_SUCCESS) {
return nullptr;
ret = RegOpenKeyExW(hKeyAllTimeZones, timezoneSubKeyName, 0, KEY_READ,
if (ret != ERROR_SUCCESS) {
return nullptr;
// Note: RegQueryValueExW wants the size of the buffer in bytes.
size = sizeof(registryStandardName);
ret = RegQueryValueExW(hKeyTimeZoneSubKey, L"Std", nullptr, &registryValueType,
reinterpret_cast<LPBYTE>(registryStandardName), &size);
if (ret != ERROR_SUCCESS || registryValueType != REG_SZ) {
return nullptr;
// Note: wcscmp does an ordinal (byte) comparison.
if (wcscmp(reinterpret_cast<WCHAR *>(registryStandardName), dynamicTZI.StandardName) == 0) {
// Since we are comparing the *localized* time zone name, it's possible that some languages might use
// the same string for more than one time zone. Thus we need to examine the TZI data in the registry to
// compare the GMT offset (the bias), and the DST transition dates, to ensure it's the same time zone
// as the currently reported one.
REG_TZI_FORMAT registryTziValue;
uprv_memset(&registryTziValue, 0, sizeof(registryTziValue));
// Note: RegQueryValueExW wants the size of the buffer in bytes.
DWORD timezoneTziValueSize = sizeof(registryTziValue);
ret = RegQueryValueExW(hKeyTimeZoneSubKey, L"TZI", nullptr, &registryValueType,
reinterpret_cast<LPBYTE>(&registryTziValue), &timezoneTziValueSize);
if (ret == ERROR_SUCCESS) {
if ((dynamicTZI.Bias == registryTziValue.Bias) &&
(memcmp((const void *)&dynamicTZI.StandardDate, (const void *)&registryTziValue.StandardDate, sizeof(SYSTEMTIME)) == 0) &&
(memcmp((const void *)&dynamicTZI.DaylightDate, (const void *)&registryTziValue.DaylightDate, sizeof(SYSTEMTIME)) == 0))
// We found a matching time zone.
windowsTimeZoneName = timezoneSubKeyName;
hKeyTimeZoneSubKey = nullptr;
if (hKeyTimeZoneSubKey != nullptr) {
if (hKeyAllTimeZones != nullptr) {
CharString winTZ;
UErrorCode status = U_ZERO_ERROR;
winTZ.appendInvariantChars(UnicodeString(TRUE, windowsTimeZoneName, -1), status);
// Map Windows Timezone name (non-localized) to ICU timezone ID (~ Olson timezone id).
StackUResourceBundle winTZBundle;
ures_openDirectFillIn(winTZBundle.getAlias(), nullptr, "windowsZones", &status);
ures_getByKey(winTZBundle.getAlias(), "mapTimezones", winTZBundle.getAlias(), &status);
ures_getByKey(winTZBundle.getAlias(),, winTZBundle.getAlias(), &status);
if (U_FAILURE(status)) {
return nullptr;
// Note: Since the ISO 3166 country/region codes are all invariant ASCII chars, we can
// directly downcast from wchar_t to do the conversion.
// We could call the A version of the GetGeoInfo API, but that would be slightly slower than calling the W API,
// as the A version of the API will end up calling MultiByteToWideChar anyways internally.
wchar_t regionCodeW[3] = {};
char regionCode[3] = {}; // 2 letter ISO 3166 country/region code made entirely of invariant chars.
int geoId = GetUserGeoID(GEOCLASS_NATION);
int regionCodeLen = GetGeoInfoW(geoId, GEO_ISO2, regionCodeW, UPRV_LENGTHOF(regionCodeW), 0);
const UChar *icuTZ16 = nullptr;
int32_t tzListLen = 0;
if (regionCodeLen != 0) {
for (int i = 0; i < UPRV_LENGTHOF(regionCodeW); i++) {
regionCode[i] = static_cast<char>(regionCodeW[i]);
icuTZ16 = ures_getStringByKey(winTZBundle.getAlias(), regionCode, &tzListLen, &status);
if (regionCodeLen == 0 || U_FAILURE(status)) {
// fallback to default "001" (world)
status = U_ZERO_ERROR;
icuTZ16 = ures_getStringByKey(winTZBundle.getAlias(), "001", &tzListLen, &status);
// Note: We want the first entry in the string returned by ures_getStringByKey.
// However this string can be a space delimited list of timezones:
// Ex: "America/New_York America/Detroit America/Indiana/Petersburg ..."
// We need to stop at the first space, so we pass tzLen (instead of tzListLen) to appendInvariantChars below.
int32_t tzLen = 0;
if (tzListLen > 0) {
while (!(icuTZ16[tzLen] == u'\0' || icuTZ16[tzLen] == u' ')) {
// Note: cloneData returns nullptr if the status is a failure, so this
// will return nullptr if the above look-up fails.
CharString icuTZStr;
return icuTZStr.appendInvariantChars(icuTZ16, tzLen, status).cloneData(status);