Ask HN: How precise are computer clocks?

Generally the RTC in a computer is not as accurate as a typical wall clock or wristwatch. Although the RTC usually uses the same type of crystal as a watch, the circuit is not usually calibrated (tuned) as part of the production and test process. I have seen drift rates of up to 5-10 minutes per month on systems that aren't using NTP.

Because network-synchronized time clients are now nearly everywhere, the cost and accuracy of onboard computer clocks has declined over the past decade.

At the extreme, Raspberry Pis don't even have an integrated battery-sustained clock, and if the Pi has no network connection, it has no time at all.

> How much would its clock/time deviate, especially compared to a quartz or mechanical watch?

In my experience, quite a lot. To see this for yourself, log two machines onto http://arachnoid.com/NaturalResources/big_clock.html. On one machine, make a local copy of the page and disconnect from the Internet. See how quickly that machine deviates from correct time.