Cost: Free
The FastHabit app is perfect for any fasting method, whether you fast the same number of hours each day or if your fasting window changes. For example, it works if you do 16:8 or if you do the 5:2 method and want to have a shorter eating window on those to days.
It allows you to set the length of your fasting window, which you can edit after your window has already begun. You can also add a fast that happened on a past day. There's a clock that counts down the time until your fast ends, so at a quick glance, it can inspire you to keep a fast going when you only have a short time left.
You can also see a summary of the last 10 days, which could help motivate you to stick with your daily fasting schedule, and you can set reminders that send notifications to your phone or Apple Watch when a fast starts or ends.