system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:why do (some) digital clocks run behind/ahead after a while

I noticed my digital clock (which i use to wake up in the morning) started running ahead of the real time. I'm now trying to keep a log of how much it runs ahead, to see if it's significant. Last year it didn't have a significant difference between displayed time and actual time. (The clock was plugged out for a month or two)

So I wanna know why they can run ahead or behind in time.
gpt
Digital clocks work thanks to Quartz crystal oscillation.

Basically, if you take a quartz crystal and pass an electronic current through it, it will oscillate at a specific frequency. 

Most work at about 32,768hz, which is 2^15 cycles per second. Basically, a computer chip counts the oscillations. Every 32,768 oscillations, the clock moves forward one second.

This is very accurate and reliable, generally speaking. The issue is, pressure and temperature affect how fast the crystal oscillates. A 10C difference can cause about 2min/year to be lost, a 20C difference can be a difference of 10min/year. If your unit gets hot, or sits in a hotter than normal area (like over a heater, on a sunny window sill), it can have a noticeable difference.