late late late !!

CMOS = BATTERY on Motherboard
It's usually a small flat circular battery, like you find in cameras etc..

CMOS Batteries are Volatile. Which basically means, you can flatten them by handling them incorrectly.
Infact just holding them in your fingers is enough

Only way to check voltage is with a multi-meter.
 
Reasons for this off the top of my head would be

Battery
Incorrect timezone on PC - that will make you a hour -/+ even if you reset
Incorrect timezone on router - if using network time it will also make your PC revert to bad date/time.
And I guess some types of malware used to do this also.
 
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