Earlier quoted context omitted.
One of my very vague goals is to build a clock, from scratch, without looking anything up. It wouldn't have to look like a wall clock; just something that keeps 24-hour time within reasonable tolerances. (It's been impossible to avoid knowledge of all clockmaking stuff, but I still avoid watching videos on how clocks are made. This is probably going to be something I do once the child is in college!)
Extra challenge: no other clocks to use as a time reference.
Come to think of it, I guess the easiest possible "mission accomplished" I could declare is by jamming a stick into the ground on a sunny weekend.