system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do clocks become fast or slow over time?

For example, I’ll set my watch to the exact time that’s on my phone and a month or so later, it’s 2-3 minutes slow.
gpt
Your watch is not keeping the exact time. There are 86400 seconds in a day, but if it is mechanically measuring 86399 seconds in a day, then after 2 months, it'll be one minute off.

We're talking about a 0.001% error here. For many inexpensive watches, it is 'easy' to not build to this level of specificity.

This is especially true since after 6 months, you'll be 3 minutes out of date at this rate, and then you change your watch for Daylight Savings time anyways, and you'll fix the error then.