Chimer

uninterrupted periodic nudge alarm app
- watch only app

once you set timer, it works even on the background. You can check out the set time on the complications.

p.s. in case you don't like Chimer app opens on wrist raise, go to watch's settings -> General -> return to clock -> Chimer -> select customize and select always on "while not in session" then it won't pop up on wrist raise.

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