What other kinds of things can period-tracker apps be used for?

I'm not familiar with how they work or what kinds of data they track and return. Would they be useful to track things like plant feeder changes, cpap cleaning schedules, ant farm maintenance...

Can you think useful alternative applications and life hacks for these apps?

edit: I know that there are alternative scheduling apps for virtually any need. That's not the point. I like finding new and useful ways to use existing things.