A Time Zone API supplement

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:

  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.
  Get the list of TimeZones
              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.
              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
              SYSTEMTIME *pEncoded          : The encoded SYSTEMTIME 
              UINT wYear                    : Year to work with
              SYSTEMTIME *pOUT              : The decoded SYSTEMTIME 
  void  WINAPI AdjustSystemTime(SYSTEMTIME &stIn, TIME_ZONE_INFORMATION  &tzi);
  Return the updated SYSTEMTIME for the timezone
              SYSTEMTIME stIn               : Time to modify
              TIME_ZONE_INFORMATION tzi     : The Time Zone Infomation block
              Updated stIn
  Return the Offset in minutes for the timezone
              SYSTEMTIME stIn               : Time to check
              TIME_ZONE_INFORMATION tzi     : The Time Zone Infomation block
              LONG                          : Offset in Minutes
  Return the DST setting for the timezone
              SYSTEMTIME stIn               : Time to check
              TIME_ZONE_INFORMATION tzi     : The Time Zone Infomation block
              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]))

int _tmain(int argc, TCHAR *argv[])
	 // Get the number of time zones
	int iCount = (int)TZIEnum(NULL, 0);
	 // Allocate the space
	 // 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
	 // 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);

	 // Cleanup
	return 0;

About the Author

Erik Gawtry

I have worked as a Software Engineer since 1985. I currently work with Windows, Unix/Linux, DSP, and Microcontrollers.



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