The date/time on your system is maintained by the CMOS Battery on your
motherboard. Generally, problems with losing time on a system can be
attributed to a failing CMOS battery. These are usually easy to replace and
are simple coin-type batteries held in place by a small spring.
Here's a good pictorial showing what I'm talking about (note - not all CMOS
batteries are held in place in the exact same fashion.)
http://www.liverepair.com/encyclopedia/articles/cmosreplace.asp
And here's a place that sells the coin-type for $3, though you should be
able to find one locally anywhere that sells watch batteries (confirm the
voltage.)
http://www.cablesnmor.com/coin-cell-batteries.html