i sometimes get lagg /slowing on my clock depending on the workload of my system !? i replaced the board battery and still the same problem !? a reboot fixed the clock !?
Alittle digging around both on the net and by chance of messing about my system i discovered the more workload i put on it the more the clock lagged and lost time ?!
So this could be your problem !? my solution was to have less background tasks and less workload running and the problem solved its self
A quick fix is a util to adjust the clock with the atomic sync clocks on the internet there are lots of freeware tools out there that do this