我正在尝试查看WindAPI从本地转换为UTC时间，反之亦然是夏令时准确。例如，我们采用LocalFileTimeToFileTime API。其描述说明：
LocalFileTimeToFileTime使用时区的当前设置 和夏令时。因此，如果是夏令时， 此功能将夏令时考虑在内，即使是 你转换的时间是标准时间。
所以我用这段代码测试它：
//Say, if DST change takes place on Mar-8-2015 at 2:00:00 AM
//when the clock is set 1 hr forward
//Let's check the difference between two times:
SYSTEMTIME st1_local = {2015, 3, 0, 8, 1, 30, 0, 0}; //Mar-8-2015 1:30:00 AM
SYSTEMTIME st2_local = {2015, 3, 0, 8, 3, 30, 0, 0}; //Mar-8-2015 3:30:00 AM
//Convert to file-time format
FILETIME ft1_local, ft2_local;
VERIFY(::SystemTimeToFileTime(&st1_local, &ft1_local));
VERIFY(::SystemTimeToFileTime(&st2_local, &ft2_local));
//Then convert from local to UTC time
FILETIME ft1_utc, ft2_utc;
VERIFY(::LocalFileTimeToFileTime(&ft1_local, &ft1_utc));
VERIFY(::LocalFileTimeToFileTime(&ft2_local, &ft2_utc));
//Get the difference
LONGLONG iiDiff100ns = (((LONGLONG)ft2_utc.dwHighDateTime << 32) | ft2_utc.dwLowDateTime) -
(((LONGLONG)ft1_utc.dwHighDateTime << 32) | ft1_utc.dwLowDateTime);
//Convert from 100ns to seconds
LONGLONG iiDiffSecs = iiDiff100ns / 10000000LL;
//I would expect 1 hr
ASSERT(iiDiffSecs == 3600); //But I get 7200, which is 2 hrs!
那我在这里错过了什么？
答案 0 :(得分：6)
SystemTimeToFileTime()将其第一个参数解释为UTC时间（没有DST概念），因此您的ft1_local和ft2_local对象将始终相隔两个小时数据格式，但不是实际的时间点。然后，LocalFileTimeToFileTime()会将相同的偏移量应用于您传递给它的任何内容，因此ft1_utc和ft2_utc总是会相隔两小时。
正如文档所说，“LocalFileTimeToFileTime 使用当前设置作为时区和夏令时”（强调我的），所以如果在当前时间你是四个小时例如，在UTC之后，它只会从您传递给它的任何时间扣除四个小时，无论该时间是否最初代表DST的另一侧。
编辑：根据评论，这里是你如何获得标准C中两个本地时间之间的秒差：
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(void) {
struct tm start_time;
start_time.tm_year = 115;
start_time.tm_mon = 2;
start_time.tm_mday = 8;
start_time.tm_hour = 1;
start_time.tm_min = 30;
start_time.tm_sec = 0;
start_time.tm_isdst = -1;
struct tm end_time;
end_time.tm_year = 115;
end_time.tm_mon = 2;
end_time.tm_mday = 8;
end_time.tm_hour = 3;
end_time.tm_min = 30;
end_time.tm_sec = 0;
end_time.tm_isdst = -1;
time_t start_tm = mktime(&start_time);
time_t end_tm = mktime(&end_time);
if ( start_tm == -1 || end_tm == -1 ) {
fputs("Couldn't get local time.", stderr);
exit(EXIT_FAILURE);
}
double seconds_diff = difftime(end_tm, start_tm);
printf("There are %.1f seconds difference.\n", seconds_diff);
return EXIT_SUCCESS;
}
输出：
paul@thoth:~/src$ ./difftime
There are 3600.0 seconds difference.
paul@thoth:~/src$
正如你所期待的那样。
请注意，使用struct tm：
tm_year表示自1900年以来的年份，所以到2015年我们写了115
tm_mon的范围是0到11，所以March是2，而不是3。
其他时间成员正如您所期望的那样
当tm_isdst设置为-1时，mktime()将尝试自行查明DST是否在我们提供的当地时间生效，这就是我们想要的要做的，这里。
答案 1 :(得分：2)
尽管Paul Griffiths＆＃39;解决方案，由于明显的区域设置限制，我无法使用它。 （C显然显示了它的年龄。）所以我不得不采用纯WinAPI方法。接下来是我想出的。如果我错了，请纠正我（尤其是那些可以访问微软mktime似乎赞成的美国以外时区的人）：
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());
}
然后这是实际的实现。这里需要注意的一个重要方面是，由于缺少用于检索特定年份的时区信息的API，以下方法可能无法在Windows XP上可靠地运行。
首先是一些声明：
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);
实施：
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;
}