"Most of the original designs did use crystals."
_"_Yes, but they don't NOW. "
This all started around "I have an Arduino Mega2560 clone (Chinese) board."
Who knows what kind of resonator part was used? Could have a crappy batch of parts to start with that no one knew enough to complain about.
Has the same test been done on a current Mega? I have run a simple time keeping sketch on an Uno, and it tracked the time against official US Gov't time pretty well, I didn't notice seconds of drift a day.