When you click the date in the top bar and go to Date & Time Preferences, you'll see a lock in the lower-left. Like just brew it! said, you need to be an administrator on the machine. Click the lock, enter admin credentials, and then change it as necessary.
You might also want to look at the time zone after you click the lock. It could be that the time is fine, but it's for a different zone.
I do not understand what I do. For what I want to do I do not do, but what I hate I do.
Twittering away the day at @TVsBen