The approximate period start date, end date, and cycle length you enter at registration dictates the days and length of your other phases. As you begin to use the app, you may notice that your other phases are a day or two off. Your continued use of the app is the fastest way to improve the accuracy of each phase length. The more you use it, the more it will be able to reflect your body's natural rhythms. The app will calculate your average phase and cycle length as you enter data over time.