Windows Time

Windows time is the number of milliseconds elapsed since the system was last started. This format exists primarily for backward compatibility with 16-bit Windows. To ensure that applications designed for 16-bit Windows continue to run successfully, the GetTickCount function returns the current Windows time.

You typically use the GetTickCount or GetTickCount64 function to compare the current Windows time with the time returned by the GetMessageTime function. GetMessageTime returns the Windows time when the specified message was created. GetTickCount and GetTickCount64 are limited to the resolution of the system timer, which is approximately 10 milliseconds to 16 milliseconds. The elapsed time retrieved by GetTickCount or GetTickCount64 includes time the system spends in sleep or hibernation.

If you need a higher resolution timer, use the QueryUnbiasedInterruptTime function, a multimedia timer, or a high-resolution timer. The elapsed time retrieved by the QueryUnbiasedInterruptTime function includes only time that the system spends in the working state.

Windows Server 2008, Windows Vista, Windows Server 2003 and Windows XP/2000:  The QueryUnbiasedInterruptTime function is available starting with Windows 7 and Windows Server 2008 R2.

You can use the System Up Time performance counter to obtain the number of seconds elapsed since the computer was started. This performance counter can be retrieved from the performance data in the registry key HKEY_PERFORMANCE_DATA. The value returned is an 8-byte value. For more information, see Performance Counters.