Peter,
You may well find that the operating system you are using has been told (by default?) to adjust for daylight saving time.
You can check by going to Start>settings>control panel, then opening the "Date/time" window, and select the "Time Zone" tab. At the bottom left there should be a tick box mentioning "automatically adjust clock for daylight saving changes"
If you live in a time zone where daylight saving is not used, you might like to check/replace the CMOS battery on the motherboard.
N.B. Check your mobo documentation BEFORE tinkering with the battery, as you will either lose all the BIOS settings by removing the battery (they revert to the factory default including passwords), OR you have a short time to swap batteries quickly before losing any data.
Hope this helps.
Best wishes.