After a week, 20 ppm would drift 12 * 10^-6 * 7 * 24 * 60 *60 = 12 seconds.
Your motherboard probably has a cr2032 keeping it powered when unplugged.
Crystals: https://www.digikey.com/en/products/filter/crystals/171?s=N4...
Really good watches allow you to adjust their rate, so if it runs slightly fast or slow at your wrist temperature, you can correct it.
One of the key insights of John Harrison, who won the Longitude prize, was that it doesn’t matter so much if a clock runs slightly fast or slightly slow, so long as it ticks at a very steady rate. Then you can characterise its frequency offset, and use that as a correction factor to get the correct GMT after weeks at sea.
Or are you saying that what makes quartz crystals drift is the change in temperature?
Still, wouldn't the temperature of a watch while being worn vary as least as much as when sitting in a drawer (unless you live in a region blessed with t-shirt weather year around)?
One of my favorite wristwatches I used to wear as a teenager had a thermometer, but I don't remember how exactly that varied over the year, just that it always showed neither quite my body temperature, nor quite the ambient one :)
Thus, the tempco is near zero, so human-to-human differences don’t matter much.
One thing to notice is that quartz watches almost always have a metal backplate touching your wrist so that the crystal can have good thermal contact. Presumably, the thermometer in your watch was decoupled from that plate.
In an alternate universe it would've been put in there, together with all the weird -12V / -5V rails nobody uses these days. Getting it these days would indeed be pretty much impossible.
More at https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html
Where are you getting that 12 from?
The end result is 12 seconds.