Getting the current time (in milliseconds) from the system clock in Windows?

Question

How can you obtain the system clock's current time of day (in milliseconds) in C++? This is a windows specific app.

Answer 1

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().

Answer 2

To get the time expressed as UTC, use GetSystemTime in the Win32 API.

SYSTEMTIME st;
GetSystemTime(&st);

SYSTEMTIME is documented as having these relevant members:

WORD wYear;
WORD wMonth;
WORD wDayOfWeek;
WORD wDay;
WORD wHour;
WORD wMinute;
WORD wSecond;
WORD wMilliseconds;

As shf301 helpfully points out below, GetLocalTime (with the same prototype) will yield a time corrected to the user's current timezone.

You have a few good answers here, depending on what you're after. If you're looking for just time of day, my answer is the best approach -- if you need solid dates for arithmetic, consider Alex's. There's a lot of ways to skin the time cat on Windows, and some of them are more accurate than others (and nobody has mentioned QueryPerformanceCounter yet).

Answer 3

A cut-to-the-chase example of Jed's answer above:

const std::string currentDateTime() {

SYSTEMTIME st, lt;

GetSystemTime(&st);

char currentTime[84] = "";

sprintf(currentTime,"%d/%d/%d  %d:%d:%d %d",st.wDay,st.wMonth,st.wYear, st.wHour, st.wMinute, st.wSecond , st.wMilliseconds);

return string(currentTime); }