Time Zone API
Unfortunately, the Windows Time Zone API only has four functions:
- GetTimeZoneInformation()
- GetTimeZoneInformationForYear()
- GetDynamicTimeZoneInformation()
- GetDynamicTimeZoneInformationForYear()
These functions only work on the local time zone and, as such, as useless for
any globalized application.
This API contains enumeration for all the World’s time zones, their bias
(offset from UTC), and both the Standard and Daylight labels for the time zones.
What is interesting is that all this information is built into Windows, but it’s
not accessable normally.
Defined Time Zone API Functions:
TZI_LOCALTIMEZONE Defined so that NO TIME ZONE is a valid setting for the Bias. Use this for the bias to indicate that there is no bias and that it is a local time. UINT WINAPI TZIEnum(TIME_ZONE_INFORMATION *pTZI, UINT cbTZI); Get the list of TimeZones Param: TIME_ZONE_INFORMATION pTZI : The Time Zone Infomation array UINT cbTZI : Size of Array to fill Note: Pass NULL and 0 to get the total size needed. Return: UINT : Count of TZI structures enumerated The structures will be filled in in the TZI array void WINAPI FindTimezoneDate(SYSTEMTIME *pEncoded, UINT wYear, SYSTEMTIME *pOut); Decode the encoded SYSTEMTIME Param: SYSTEMTIME *pEncoded : The encoded SYSTEMTIME UINT wYear : Year to work with SYSTEMTIME *pOUT : The decoded SYSTEMTIME Return: None void WINAPI AdjustSystemTime(SYSTEMTIME &stIn, TIME_ZONE_INFORMATION &tzi); Return the updated SYSTEMTIME for the timezone Param: SYSTEMTIME stIn : Time to modify TIME_ZONE_INFORMATION tzi : The Time Zone Infomation block Return: Updated stIn LONG WINAPI GetAdjustSystemTime(SYSTEMTIME &stIn, TIME_ZONE_INFORMATION &tzi); Return the Offset in minutes for the timezone Param: SYSTEMTIME stIn : Time to check TIME_ZONE_INFORMATION tzi : The Time Zone Infomation block Return: LONG : Offset in Minutes BOOL WINAPI IsDST(SYSTEMTIME &stIn, TIME_ZONE_INFORMATION &tzi); Return the DST setting for the timezone Param: SYSTEMTIME stIn : Time to check TIME_ZONE_INFORMATION tzi : The Time Zone Infomation block Return: BOOL : DST setting for the passed time
Sample Code:
/* * Sample program to demonstrate the Time Zone API * * Copyright (c) 2011 Erik H Gawtry */ #include <stdio.h> #include <windows.h> #include <tchar.h> #include "TimeZone.h" #ifndef countof #define countof(a) (sizeof(a)/sizeof(a[0])) #endif int _tmain(int argc, TCHAR *argv[]) { // Get the number of time zones int iCount = (int)TZIEnum(NULL, 0); // Allocate the space TIME_ZONE_INFORMATION *pTZI = (TIME_ZONE_INFORMATION *)malloc(sizeof(TIME_ZONE_INFORMATION)*(iCount+1)); // Zero out ZeroMemory(pTZI, sizeof(TIME_ZONE_INFORMATION)*(iCount+1)); // Get the time zone entries TZIEnum(pTZI, iCount); SYSTEMTIME st, stDaylightDate, stStandardDate; TCHAR szDaylightDate[64], szStandardDate[64]; // Get the UTC system time GetSystemTime(&st); // Loop for(int i = 0; i < iCount; ++i) { // Is it DST? BOOL bDST = IsDST(st, pTZI[i]); // What is the offset LONG lBias = GetAdjustSystemTime(st, pTZI[i]); bool bNegative = (lBias<0); lBias = labs(lBias); // Display the entry with offset _tprintf(_T("%c(%02d:%02d): %-32.32s (%s)"), (bNegative)?'-':'+', lBias/60, lBias%60, bDST?pTZI[i].DaylightName:pTZI[i].StandardName, bDST?_T("DST"):_T(" ST") ); // DST specified for the time zone? if( pTZI[i].DaylightDate.wMonth ) { // Convert to normal time FindTimezoneDate(&pTZI[i].DaylightDate, st.wYear, &stDaylightDate); FindTimezoneDate(&pTZI[i].StandardDate, st.wYear, &stStandardDate); // Format for output GetDateFormat(LOCALE_USER_DEFAULT, DATE_SHORTDATE, &stDaylightDate, NULL, szDaylightDate, countof(szDaylightDate)); GetDateFormat(LOCALE_USER_DEFAULT, DATE_SHORTDATE, &stStandardDate, NULL, szStandardDate, countof(szStandardDate)); _tprintf(_T(" %-10.10s %-10.10s"), szDaylightDate, szStandardDate); } _tprintf(_T("n")); } // Cleanup free(pTZI); return 0; }