Sometimes the realtime clocks were based on watch movements, ok, I only know of one instance, but it is fascinating. these movements were purpose built as a low power embedded device that does one thing well, drive a seven segment display to show the time. so they had to jump through some wild hoops to get it to play with the rest of the system.

https://www.youtube.com/watch?v=pr6HTiWrMmk (CuriousMarc)