Ive used this app across 3 generations of iPhone and its been a lifesaver. Super simple to use, gives me a heads up when Im about to start, and lets you track all sorts of stuff like weight loss/gain, ovulation & fertility, and so much more (even at the free stage, which is what I use). Definitely considering the paid upgrade, which allows you to backup your data and transfer it from phone to phone--something Ive been missing all these years! 5/5 would, and do, recommend.
twizzers about Period Tracking Calendar, v9.0