system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how does a computer know how long a second is?

How does a computer know exactly how long a second is? Does it know it as a rhythm? Does it know it by a certain number of computer thingys to equal one second (like unit conversion)?
gpt
There is a clock on the motherboard driven by a quartz crystal that vibrates at a fixed frequency. That is connected to a counter chip that simply tots up the number of pulses from the crystal and sends out a pulse of its own when the count reaches a certain value, the number of pulses in a second. It's just the same as the way billions of digital clocks work.

It's the reason that there is a small battery on the motherboard, to power the time keeping circuit even when the power is off.