My Snow Leopard macbook is set to
Set date and time automatically: Apple Americas/U.S. (time.apple.com), but despite sitting idle for several hours (with internet connection), it has yet to do any syncing, the date remains off by ~2 days.
ntpdate man page, the accepted answer will become obsolete when
ntpdate is retired. If you encounter this problem, this will do the equivalent:
sudo ntpd -q
ntpd: ...sets and maintains the system time of day in syn- chronism with Internet standard time servers.
-q: Set the time and quit.
you can go into the system preferences and reset the time manualy, and then set the time to sync automaticly again once you are back up to speed with whatever day/week/month you need. i am not sure why its not syncing with apple, but you can just set the day properly and it will be accurate as long as you dont cross too many time zones.
The currently accepted answer does NOT work in Mojave, as ntpdate and co have been removed.
See this excellent answer on the Apple StackExchange for up-to-date instructions.