Reboot the PC in the evening, then go into the BIOS
and check the time. Leave the PC running while in the
BIOS. Check the time again next morning, before you
reboot the PC. Is the time correct or not?
Does the machine only loose time when it is turned off?
If so then you may want to replace the motherboard battery.
As a work around you could download and install a time
sync. program such as Tardis 2000. This will update your
PC's clock with the atomic Internet clock to save you
having to manually adjust the time every day.