now() time off

Earlier this week when I logged into my database, I just so happened to
notice that the value for now() was six hours off of the time reported
by the operating system when using the date command from the command
line.  How is this possible?  I rebooted the server and the problem
magically went away.  I am quite certain that I didn't modify anything
in the database that should cause this problem.  It turns out that that
the now() time has been off for some time, and resulted in quite a bit
of corrupted data.  What exactly should I do in the future to keep this
from happening, or where should I look for clues?

John