Question

How can I set the CMOS clock?

Answer

Basically ntpd only sets the system time of the operating system. Therefore setting the CMOS clock is the responsibility of the operating system and its associated tools. To make things worse, typical PC operating systems and the BIOS set the RTC to local time, while UNIX-like operating systems set the RTC to UTC.

In Linux the RTC is either not updated at all, or just the minutes and seconds. The related kernel code has been revised several times, with different behaviour. Setting the system time manually does not update the RTC. Only if the STA_UNSYNC bit is cleared, the kernel will periodically update the RTC from the system time. Typically this happens every 11 minutes.

With the optional PPSkit-0.9.0 kernel patch the RTC is updated just like in other PC operating systems. In addition the automatic periodic update can be disabled completely (see the documentation that comes with the product).

There is also a user-space program to set the RTC, but it requires special privileges. Typically the utility is named hwclock.

-- SteveKostecke - 02 May 2003