There is a bug with the Windows 8 Taskbar in that, after a period of time, the taskbar will refuse to update itself. It stays persistently frozen and/or out of sync, including the clock, running application window icons, and system tray icons. Most people will probably notice the clock first, since they may be surprised by the time it reports.
The taskbar will do a one time update when one switches to the Modern UI Start screen.
Restarting explorer will fix the issue for a time.
The cause is unknown. This issue has been reported by many, but no acknowledgement from Microsoft is known.