It feels kind of weird to write up an item that keeps less than perfectly accurate track of time.
I mean, take a 1/day item of a 0th-level spell. It costs 180 gp, and it always knows when
exactly 24 hours have passed. You even have some items like Healing Belt, which have an "every day at dawn" clause, meaning it always know exactly when dawn is, even if it was created in the Underdark and has never once been anywhere near the surface.
I believe a magic timepiece should have all the following characteristics:
- A hand on the main dial showing the position of the sun (straight up at noon, straight left at sunset, straight down at midnight, and straight right at sunrise)
- A hand on the main dial showing the position of the moon
- A small inset dial for the phase of the moon
- Ability to keep accurate time forever inside a dungeon
- Ability to adjust itself if taken to a different part of the planet
- Ability to change how many dials of each type it has if taken to a different plane
- Very low cost