windows上获得高精度时间戳 GetSystemTimeAsFileTime

如果你需要高精度时间戳（绝对时间点），而不是计算相对时间，那么可以考虑这个API:

GetSystemTimeAsFileTime()

该函数返回自1601年1月1日起，到现在的时间（以 100ns 为单位）。

引用自： https://stackoverflow.com/questions/1695288/getting-the-current-time-in-milliseconds-from-the-system-clock-in-windows

The easiest (and most direct) way is to call GetSystemTimeAsFileTime(), which returns a FILETIME, a struct which stores the 64-bit number of 100-nanosecond intervals since midnight Jan 1, 1601.

At least at the time of Windows NT 3.1, 3.51, and 4.01, the GetSystemTimeAsFileTime() API was the fastest user-mode API able to retrieve the current time. It also offers the advantage (compared with GetSystemTime() -> SystemTimeToFileTime()) of being a single API call, that under normal circumstances cannot fail.

To convert a FILETIME ft_now; to a 64-bit integer named ll_now, use the following:

ll_now = (LONGLONG)ft_now.dwLowDateTime + ((LONGLONG)(ft_now.dwHighDateTime) << 32LL);

You can then divide by the number of 100-nanosecond intervals in a millisecond (10,000 of those) and you have milliseconds since the Win32 epoch.

To convert to the Unix epoch, subtract 116444736000000000LL to reach Jan 1, 1970.

You mentioned a desire to find the number of milliseconds into the current day. Because the Win32 epoch begins at a midnight, the number of milliseconds passed so far today can be calculated from the filetime with a modulus operation. Specifically, because there are 24 hours/day * 60 minutes/hour * 60 seconds/minute * 1000 milliseconds/second = 86,400,000 milliseconds/day, you could user the modulus of the system time in milliseconds modulus 86400000LL.

For a different application, one might not want to use the modulus. Especially if one is calculating elapsed times, one might have difficulties due to wrap-around at midnight. These difficulties are solvable, the best example I am aware is Linus Torvald’s line in the Linux kernel which handles counter wrap around.

Keep in mind that the system time is returned as a UTC time (both in the case of GetSystemTimeAsFileTime() and simply GetSystemTime()). If you require the local time as configured by the Administrator, then you could use GetLocalTime().

例如，想要获得从1601年1月1日以来，到目前为止所经历过的 微秒(micro seconds)数，可以这样做：

FILETIME fileTime;
long long now_long;
GetSystemTimeAsFileTime(&fileTime);
now_long = (long long(fileTime.dwHighDateTime) << 32) + fileTime.dwLowDateTime;
std::cout << "micro time from 1601.1.1:00:00:00 is: " << (now_long / 10) << std::endl;