One morning I turned on my Mac and found the date mysteriously set back to 2000. iCal was displaying a message indicating the option to set the date automatically. Do I have a virus or have I been hacked. I use Mac-Keeper but is that enough?
The date being reset to January 1st, 2000 (or Jan. 1st 1970 on older Macs) usually indicates a dead PRAM Battery but here are all three options:
1. your PRAM was reset (via Terminal or by pressing cmd+alt+P+R during bootup)
2. your PRAM is corrupt (in that case you should do the PRAM reset as above)
3. your PRAM Battery is dead
Since replacing the PRAM Battery is expensive (+40$ excl. work time) I suggest to do a PRAM Reset and see if it happens again. Of course if you have extended warranty or Apple Care that's still valid it should be covered.
I don't know whether this is the same with Macs, but if a PC suddenly starts 'losing' its time, then this is a strong indication that the CMOS Battery on the motherboard is going low and needs to be replaced. Soon after the date going awry, the computer will forget its BIOS settings and boot into the BIOS, asking for basic settings, before booting the operating system.
As I said, I can only speak for Windows machines, but Macs also have a CMOS battery. See this article: https://discussions.apple.com/thread/1908953?threadID=1908953
More info about the topic: 3 Reasons Why Your Windows Computer Clock Loses Its Time