dell optiplex 745 time will not stay curent.

I have a problem that I seem I can not find a answer to. I have a Dell Optiplex 745 that in winXP the time keeps reverting back to 1980. I have replaced the CMOS battery on the motherboard. That did not help. Dell has replaced the Motherboard. That did not help. Any ideas? Updates are all there everything else seems normal. I can change the time and with in a few mins it’s back to 1980. The BIOS clock was setting it’s self to 2156, but after dell replaced the Board it has stayed the same. In total I have about 4 optiplex 745 doing this. Anti virus is up to date. I have also reformatted one of the 745 and it started having the same problem after I reformatted it.