system
Given the following post, respond as if you're commenting on Reddit:
human
How can a digital clock become “fast”?

The digital clock in my car will slowly get more and more ahead unless I reset it; how does this happen? How do digital clocks normally function and why would it be fast of all things?
gpt
Digital clocks that plug into a wall usually use the oscillation of the AC power coming from the grid to keep track of time. This oscillation is usually very steady and predictable so it works well.

Your car isn't plugged into the grid, so it needs to keep time some other way. Often a quartz clock is used. This clock runs a current through a quartz crystal which causes it to vibrate. This vibration is pretty much constant for a given voltage so you can count the back and fourth movement to keep time.

The reason it might be running fast is because the vibration isn't perfect and there are a lot of variables. Temperature and voltage both have an effect on it, so a small difference from what the clock expects in either of these will cause it to run fast or slow. Also since these clocks are mass produced if the quality control isn't great you'll get quartz clocks that vibrate at slightly different speeds from what they are supposed to. If you don't calibrate the clock to use the actual speed it will drift as well.