Not Reliable

This is a glorified version of the Rhythm Method. It will take your longest cycle and make that the cycle length for all periods going forward. In the event that your current month will be a 36 day cycle (4 days more than ever before), it will change that months peak day by almost a week! And you wont know until after you get your period. Not good if trying to avoid pregnancy (or get pregnant too). It gives you a place to chart temperature and mucus, but that does not change the projected peak day.
Its kind of a fun app to pay with, but if youre relying on it for NFP I would NOT recommend it.

PRMiller76 about Fertility Clock: menstrual period + ovulation log, v5.1