Serene Fasting tracker is the best app to track your fasts.
It is good for window and intermittent fasting too.
If you want to have the best tool to keep track of your fasts, this app is the right choice​.

Serene supports:
- Custom Fasting times
- Analytics
- Add fasts manually
- Export data to PDF
- Simple Timer
- Reminders
- Edit your previous fasts
- Today Widget
- Watch app

This app optionally integrates with the Health App to track your weight.
It is an opt-in feature that's accessible from the logs screen.