A: This is caused by the /etc/adjtime file which is used to adjust the system clock's drift if you don't use the NTP protocol for example to obtain the system date/time from a time server.

Technical background: On systems with a reliable clock reference (such as the TSC register in Pentium-class CPUs) the difference between the elapsed time of the CPU and the system clock is saved into the /etc/adjtime file. This is used at the next reboot to adjust the system clock.

If you manually change the system date/time in the BIOS you actually confuse this mechanism: A large drift value will be written into /etc/adjtime which will cause your system clock to become wrongly adjusted each time you reboot.

Solution:

1. Check if your system clock is kept in UTC (Universal Coordinated Time). On SuSE Linux for example, this is indicated with the GMT= variable in /etc/rc.config: If GMT="-u" your system clock is kept in UTC.
2. Delete the /etc/adjtime file.
3. Use the date command to adjust the system's date/time (date MMDDhhmm[[CC]YY][.ss] - see man date).
4. Execute hwclock --systohc to write this adjusted date/time into the system clock (if your system clock is kept in UTC you should additionally specify the --utc option).