Time not being updated on my laptop?

This is a recent problem - as of the last couple of months, when I turn off my laptop or put it in standby, the time does not update when I turn it back on. That is to say, it displays the same time/date as the last time I had it on. I updates while working on it, and I can go in and change the date/time, but it never fails, the next time I turn it on, it displays the last time I had it on. I am working in Windows XP Media, I've had the laptop for about 3 years now, and this only started a month or two ago - you wouldn't think it would be frustrating, but it really is!! How can I fix it?