From my experience in building perhaps 300 PC-type computers, I have never found a clock on any motherboard that was accurate, power on or off, to more than 30 seconds a day. It just seems to be part of the hardware design.
And it always seemed odd, since 25 years ago, I built a S-100 circuit board with a $2 off-the-shelf clock IC that kept time to within a few seconds a year under battery power.
But my PC-building experience is not recent, and realtime motherboard clocks may have improved over the last 5 years.