This can (and usually does) happen when the hardware clock dies, or else when the hardware clock was accidentally set in the far future sometime in the past (and has since been brought back into line). The former is far, far more common than the latter.
Make sure the machine's system and hardware clocks are both accurate now (run
hwclock), then take the machine down in a maintenance, turn it off, pull it's power (physically disconnect it from the mains), wait a couple of minutes, then start it up again. Jump into the BIOS and check the time there. If it's still correct, then it's most likely to be a mis-set hardware clock and it probably won't happen again. If it's now wrong (probably set to Jan 1, 1988 or some other "round" time), the CMOS battery has died and you should replace it before setting the time correctly via the BIOS and booting the machine again. Keeping spare BIOS batteries around (our DC toolboxes each have a box of them) is always a good idea.