Comment by MathMonkeyMan

I forgot about the AC mains :)

My experience with embedded stuff is very limited -- just some hobby projects on the Pico.

From what I've read, though, the on-board oscillators (there are a few of them) can drift quite a bit. One data point [says][1] several seconds per day.

On the other hand, if what you want is a microcontroller with an accurate clock, they probably make those (just not the Pico).

[1]: https://forums.raspberrypi.com/viewtopic.php?t=29255