Fasting Tracker lets you retroactively edit a fast (like if you forgot to start/stop it) but wouldn’t do it automatically. I don’t know of any app that just logs them. Maybe a streak/habit forming app would be more useful?
I’ve just tried a couple habit tracking apps (android) for anyone interested:
Loop - Has a minimalist interface, let’s you respond to a personalized notification with yes, no, and later (snooze) directly from the notification. The notification doesn’t actually pop up, just shows up on the notification bar so you’d have to pull it down manually unless you mess with the settings. Still better than opening and navigating an app imo. You can also add skip days and it has graph and a completion checkmark widgets. Couldn’t get it to pop on the lockscreen for some reason. Also days can be extended to 3am which is a huge plus for me personally! If you want to get real fancy you can set measurable goals, good for water and exercise so I’m going with this one. Oh, I did get measurable goals to pop on the lockscreen
HabitHub - A more cluttered Loop, somehiw with less features. But it pops without messing with the settings. You can respond with done, fail, and skip. Also random features seem to be locked behind a premium but if you’re using it for fasting none of them are really necessary. It has some good widgets but some of them are locked by premium??? I do like that you can set a number of days to complete an it shows you a progress percentage when you click done. Works from the lock screen.