For the paranoid among us: I've heard a lot of complaints about the recent Zero update that required people to register using an email and now has what appears to be tracking and targeted advertising. Otherwise, people seem to like the interface. Other people seem to like Fastient, particularly those that like to join groups. I use Fast Habit. The basic version of that is free, but if you want to track more than 10 days or track your weight against your fasting time, you'll need to upgrade for $2.99. No registration is required and there is no advertising even in the free version.