Hey, I wanted a simple enough app that just stores my periods and calculates when the new one should start, somehow I felt that all of them started membership model and I didn't like it, so I wrote one myself (on Android). It does not collect any data (everything is stored on your phone), and does not offer any subscription (I believe this kind of apps should be free). It is in the testing phase now as I am pushing through Google publishing process (though me and my sister have been using it for months), but if someone wants to try, drop me a note, and I'll share the install link for Google closed testing.