I have a dual-booting system for switching between Windows and Ubuntu. I recently switched to Windows and back, and now my system clock in Ubuntu is incorrect. It is currently 7pm, but it displays 2pm. I don't want to set it manually, and I can't see how to make it update to the correct time.

How can I fix this?

It would also be nice if anyone could explain how to prevent this from happening again. In Windows I can easily tell it to update the time if it's off, but I don't know how to make sure it doesn't happen in Ubuntu anymore.