alt Hacker NewsI was going to mention an Atomic Clock. I use one in an RV and it sets itself even outside of cell phone range. I know it uses a radio signal of some type but I have no idea how it works. It rights itself within a few minutes of time changes, battery loss, or the like.
It's fascinating reading if you're interested in finding out the details: https://en.wikipedia.org/wiki/WWV_(radio_station)