Robert Macy wrote:

Check the motherboard's on-board battery. Maybe it's time for
a newone.

Just remembered this. When powered OFF [by means of a power strip
switch], the TIME/DATE clock keeps working, so may not be the
battery.

But battery is over 10 years old.

When you turn off any desktop PC (desktop PC made since about 1999) the
PC's power supply normally remains connected to wall power (120/240
volts). The power supply doesn't totally shut-down - it supplies a very
small amount of current to part of the motherboard the contains the CMOS
settings and clock, and the clock is powered by that current (not by the
battery).

If the computer is connected to main power through a power strip, and if
you are in the habbit of turning the power strip off when the computer
is not in use, then the on-board clock will be powered by the
motherboard battery.

So bottom line - the battery will run down faster when the computer's
power supply is not connected to a live power source.

And yes, these batteries will drain themselves down internally over time
regardless.

It's probably type CR2032 - 3 volts. Looks like a shiney new quarter.