Are you using Notes R5? and did his problem begin on or near the daylight savings time change?
If so, you may be having a problem similar to the one we experienced here. Our server is an IBM AS/400, so there may be differences in the settings with any other kind of server.
What we eventually found out is that we had not defined the DST settings on the server properly, so after every time change, folks would complain about their clocks being messed up. While this problem persisted, attempts to use the client-side clock to correct the issue resulted in the Notes clock being pushed forward more and more, even a month forward in one case. However, many of the clocks were untouched and somehow kept the correct time.
Correcting the DST info on the server and rebooting it over the weekend allowed us to bring all clocks back to the correct time.