How old is the computer? May be time to change the motherboard battery. It looks like a watch battery in a holder on the motherboard. This battery supplies power to the clock/time module chipset.
Did you try going thru the control panel to the date/time settings and reset it? It could be a bad clock/time chipset on the motherboard. XP’s not causing the problem. If it is a software problem, it would be in the BIOS settings or version. You might check to see if you have the latest BIOS version for your motherboard.
You probably have the incorrect time zone set. If this is so and you reset the time. the next time you turn your pc on it sees that you time is incorrect for your time zone and fixes itself. To check it double click the displayed time on your screen in the lower right corner and click the timezone tab on the top. Set it to the correct timezone and your problems should go away…