System Clock Appears to Lose About One Second per Hour

• Microsoft Windows NT operating system version 3.1
• Microsoft Windows NT Advanced Server version 3.1

SYMPTOMS

If the time is continuously read on an x86-based Windows NT machine, the system appears to lose a second each hour.

CAUSE

The Real Time Clock (RTC) in x86-based computers has a resolution of one second. Therefore, the Hardware Abstraction Layer (HAL) on x86 platforms uses an interval timer to increase the resolution of the system clock into the millisecond range. Windows NT's calculation of real time based on this interval timer is inaccurate enough that it could get out of sync by over a second every hour.

RESOLUTION

The calculations to determine the correct time for the system clock based on the interval timer has been improved so that it remains much closer in sync with the RTC. The system clock is still resynchronized every hour but the size of the adjustment is negligible (at most, a few milliseconds).

This was fixed within HAL.DLL for x86 based systems only. This is not a concern on MIPS or Alpha platforms.