Computer Time/Date Changes to December 31, 2000????

Okay this is really weird. I opened up my MacBook Pro after it'd been in sleep mode and the display wouldn't show. So I held down the power button, booted it back up, and my date was set to December 31, 2000, and the clock was about an hour behind. It had also forgotten my password for the network I'm usually logged into.

What kind of issue is this? Is it just a freak occurrence of should I be expecting this to happen again?