Переход на летнее и UTC-местное время с помощью WinAPI

Несмотря на всю красоту Пол Гриффитс«Решение, я не могу использовать его из-за очевидного ограничения локали. (Очевидно, C показывает свой возраст.) Поэтому мне пришлось использовать чистый подход WinAPI. Следующее, что я придумал. Поправьте меня, если я ошибаюсь (особенно люди, имеющие доступ к часовым поясам, отличным от американского, который Microsoft mktime вроде бы в пользу)

SYSTEMTIME st1 = {2015, 3, 0, 8, 1, 30, 0, 0};    //Mar-8-2015 1:30:00 AM
SYSTEMTIME st2 = {2015, 3, 0, 8, 3, 30, 0, 0};    //Mar-8-2015 3:30:00 AM

LONGLONG iiDiffNs;
if(GetLocalDateTimeDifference(&st1, &st2, &iiDiffNs))
{
_tprintf(L"Difference is %.02f sec\n", (double)iiDiffNs / 1000.0);
}
else
{
_tprintf(L"ERROR (%d) calculating the difference.\n", ::GetLastError());
}

Тогда это фактическая реализация. Здесь важно отметить один важный аспект: метод, описанный ниже, может не работать надежно в Windows XP из-за отсутствия API для получения информации о часовом поясе за определенный год.

Сначала несколько объявлений:

enum DST_STATUS{
DST_ERROR = TIME_ZONE_ID_INVALID,           //Error
DST_NONE = TIME_ZONE_ID_UNKNOWN,            //Daylight Saving Time is NOT observed
DST_OFF = TIME_ZONE_ID_STANDARD,            //Daylight Saving Time is observed, but the system is currently not on it
DST_ON = TIME_ZONE_ID_DAYLIGHT,             //Daylight Saving Time is observed, and the system is currently on it
};

#define FILETIME_TO_100NS(f) (((LONGLONG)f.dwHighDateTime << 32) | f.dwLowDateTime)

BOOL GetLocalDateTimeDifference(SYSTEMTIME* pStBegin_Local, SYSTEMTIME* pStEnd_Local, LONGLONG* pOutDiffMs = NULL);
BOOL ConvertLocalTimeToUTCTime(SYSTEMTIME* pSt_Local, SYSTEMTIME* pOutSt_UTC = NULL);
DST_STATUS GetDSTInfoForYear(USHORT uYear, TIME_ZONE_INFORMATION* pTZI = NULL);

И реализация:

BOOL GetLocalDateTimeDifference(SYSTEMTIME* pStBegin_Local, SYSTEMTIME* pStEnd_Local, LONGLONG* pOutDiffMs)
{
//Calculate difference between two local dates considering DST adjustments between them
//INFO: May not work correctly on Windows XP for a year other than the current year!
//'pStBegin_Local' = local date/time to start from
//'pStEnd_Local' = local date/time to end with
//'pOutDiffMs' = if not NULL, receives the difference in milliseconds (if success)
//RETURN:
//      = TRUE if success
//      = FALSE if error (check GetLastError() for info)
BOOL bRes = FALSE;
LONGLONG iiDiffMs = 0;
int nOSError = NO_ERROR;

if(pStBegin_Local &&
pStEnd_Local)
{
//Convert both dates to UTC
SYSTEMTIME stBeginUTC;
if(ConvertLocalTimeToUTCTime(pStBegin_Local, &stBeginUTC))
{
SYSTEMTIME stEndUTC;
if(ConvertLocalTimeToUTCTime(pStEnd_Local, &stEndUTC))
{
//Then convert into a more manageable format: FILETIME
//It will represent number of 100-nanosecond intervals since January 1, 1601 for each date
FILETIME ftBeginUTC;
if(::SystemTimeToFileTime(&stBeginUTC, &ftBeginUTC))
{
FILETIME ftEndUTC;
if(::SystemTimeToFileTime(&stEndUTC, &ftEndUTC))
{
//Now get the difference in ms
//Convert from 100-ns intervals = 10^7, where ms = 10^3
iiDiffMs = (FILETIME_TO_100NS(ftEndUTC) - FILETIME_TO_100NS(ftBeginUTC)) / 10000LL;

//Done
bRes = TRUE;
}
else
nOSError = ::GetLastError();
}
else
nOSError = ::GetLastError();
}
else
nOSError = ::GetLastError();
}
else
nOSError = ::GetLastError();
}
else
nOSError = ERROR_INVALID_PARAMETER;

if(pOutDiffMs)
*pOutDiffMs = iiDiffMs;

::SetLastError(nOSError);
return bRes;
}

BOOL ConvertLocalTimeToUTCTime(SYSTEMTIME* pSt_Local, SYSTEMTIME* pOutSt_UTC)
{
//Convert local date/time from 'pSt_Local'
//'pOutSt_UTC' = if not NULL, receives converted UTC time
//RETURN:
//      = TRUE if success
//      = FALSE if error (check GetLastError() for info)
BOOL bRes = FALSE;
SYSTEMTIME stUTC = {0};
int nOSError = NO_ERROR;

if(pSt_Local)
{
//First get time zone info
TIME_ZONE_INFORMATION tzi;
if(GetDSTInfoForYear(pSt_Local->wYear, &tzi) != DST_ERROR)
{
if(::TzSpecificLocalTimeToSystemTime(&tzi, pSt_Local, &stUTC))
{
//Done
bRes = TRUE;
}
else
nOSError = ::GetLastError();
}
else
nOSError = ::GetLastError();
}
else
nOSError = ERROR_INVALID_PARAMETER;

if(pOutSt_UTC)
*pOutSt_UTC = stUTC;

::SetLastError(nOSError);
return bRes;
}

DST_STATUS GetDSTInfoForYear(USHORT uYear, TIME_ZONE_INFORMATION* pTZI)
{
//Get DST info for specific 'uYear'
//INFO: Year is not used on the OS prior to Vista SP1
//'pTZI' = if not NULL, will receive the DST data currently set for the time zone for the year
//RETURN:
//      = Current DST status, or an error
//        If error (check GetLastError() for info)
DST_STATUS tzStat = DST_ERROR;
int nOSError = NO_ERROR;

//Define newer APIs
DWORD (WINAPI *pfnGetDynamicTimeZoneInformation)(PDYNAMIC_TIME_ZONE_INFORMATION);
BOOL (WINAPI *pfnGetTimeZoneInformationForYear)(USHORT, PDYNAMIC_TIME_ZONE_INFORMATION, LPTIME_ZONE_INFORMATION);

//Load APIs dynamically (in case of Windows XP)
HMODULE hKernel32 = ::GetModuleHandle(L"Kernel32.dll");
ASSERT(hKernel32);
(FARPROC&)pfnGetDynamicTimeZoneInformation = ::GetProcAddress(hKernel32, "GetDynamicTimeZoneInformation");
(FARPROC&)pfnGetTimeZoneInformationForYear = ::GetProcAddress(hKernel32, "GetTimeZoneInformationForYear");

TIME_ZONE_INFORMATION tzi = {0};

//Use newer API if possible
if(pfnGetDynamicTimeZoneInformation &&
pfnGetTimeZoneInformationForYear)
{
//Use new API for dynamic time zone
DYNAMIC_TIME_ZONE_INFORMATION dtzi = {0};
tzStat = (DST_STATUS)pfnGetDynamicTimeZoneInformation(&dtzi);
if(tzStat == DST_ERROR)
{
//Failed -- try old method
goto lbl_fallback_method;
}

//Get TZ info for a year
if(!pfnGetTimeZoneInformationForYear(uYear, &dtzi, &tzi))
{
//Failed -- try old method
goto lbl_fallback_method;
}
}
else
{
lbl_fallback_method:
//Older API (also used as a fall-back method)
tzStat = (DST_STATUS)GetTimeZoneInformation(&tzi);
if(tzStat == DST_ERROR)
nOSError = ::GetLastError();
else
nOSError = ERROR_NOT_SUPPORTED;
}

if(pTZI)
{
*pTZI = tzi;
}

::SetLastError(nOSError);
return tzStat;
}