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

by

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().

More Related Contents:

  1. HOW to understand custom drawn ? Which tools can I use? [closed]
  2. How to debug heap corruption errors?
  3. Qt5 Static Build yields Failed to load platform plugin “windows”
  4. MSVCP140.dll missing
  5. How to Detect the Number of Physical Processors / Cores on Windows, Mac and Linux
  6. Exporting a C++ class from a DLL
  7. What is the proper OpenGL initialisation on Intel HD 3000?
  8. C++ system() not working when there are spaces in two different parameters
  9. Sharing a global/static variable between a process and DLL
  10. Multithreaded Rendering on OpenGL
  11. How can I hook Windows functions in C/C++?
  12. What is the difference between WM_QUIT, WM_CLOSE, and WM_DESTROY in a windows program?
  13. CMake linking against shared library on windows: error about not finding .lib file
  14. Windows C++ compiler with full C++11 support (should work with Qt) [closed]
  15. Using CreateWindowEx to Make a Message-Only Window
  16. How do I get the application data path in Windows using C++?
  17. Get current username in C++ on Windows
  18. Wrote to a file using std::wofstream. The file remained empty
  19. Windows Unicode C++ Stream Output Failure
  20. How to download a file with WinHTTP in C/C++?
  21. WinAPI Sleep() function call sleeps for longer than expected
  22. Best method for storing this pointer for use in WndProc
  23. Get HWND on windows with Qt5 (from WId)
  24. Fatal error: “No Target Architecture” in Visual Studio
  25. Boost linker error: Unresolved external symbol “class boost::system::error_category const & __cdecl boost::system::get_system_category(void)”
  26. How to tame the Windows headers (useful defines)?
  27. “APIENTRY _tWinMain” and “WINAPI WinMain” difference
  28. How do I get at the exception information when using MiniDumpWriteDump out-of-process?
  29. System() calls in C++ and their roles in programming
  30. Globbing in C++/C, on Windows

Leave a Comment