Your battery is dead.  On the UNIX PC motherboard there is a
3.0 VDC lithium battery which keeps the real-time clock operating.
Unfortunately, the battery is soldered to the motherboard in most
cases.  It's either the round cylindrical canister type, or more
commonly the flat disc-shaped battery.  A suitable replacement can be
purchased at Radio Shack (BR-2325), but you'll need to get the disc
battery holder.  If you desolder the battery from the motherboard,
solder in the holder, future replacement of this battery will be much
simpler.  If you are nervous about touching a soldering iron to your
precious machine, ASCRC will do it for you (see above).