To solidify Jim's implication,

Much more solid, thanks!

is it possible that your domain controller (assuming you have one) has its clock off? Or maybe that its time zone is wrong? Or maybe that its time zone is off and its clock is wrong by three minutes?

Windows has this neat feature of being smarter than you, and part of that is distributing its time from the domain controller (and possibly the browse master, in a domain-less environment)

Yes, what Doug describes makes it sound like there are two different time domains -- like one controller with the correct time zone and another with the wrong time zone -- and that the client computers are switching back and forth. With a strict domain controller setup, you'd think that couldn't happen, but with workgroup/browser setup, who knows??