"You can set your watch to [something or someone]..." is a well-known, though somewhat old-fashioned, expression meaning that [something or someone] is extremely reliable. Sometimes it literally means punctual, but it can be used metaphorically to mean more generally reliable outside of simple punctuality.
I say it's old-fashioned because if you google the phrase, the vast majority of the top results all point to mentions of one particular episode of the TV show "The Simpsons", where Grandpa Simpson (a character portrayed as old-fashioned and conservative) says that a football player with a crew-cut has "a haircut you can set your watch to." That's a deliberately absurd and comedic use of this phrase, and it's deliberately given to the "grandpa" character.
As mechanical watches that have to be regularly adjusted become more and more rare, I expect that this expression will fall further into disuse.