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.

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;
}