Perimon is a contraception and family planning system. Who knows might come in handy sometime.

What is Perimon? Perimon is a contraception and family planning system for women from 18 to 45 years of age, that is easy to use and reliable (it has successfully been used for contraception since 1996). Perimon is a menstrual calendar which gives you insight into your cycle
What is the difference between Perimon and other cycle calendars? To optimise its forecasts Perimon calculates – in contrast to many other products – your actual cycle length and ovulation date. Furthermore Perimon was designed with the focal point on safety. Whereas many other calendars only display statistical fertility values, Perimon's fertility data are based upon the analysis of your individual cycle
Is birth control with Perimon safe? It is. If you use Perimon only with the rhythm method, we estimate the Pearl index by comparision to 3.8 (a good safety). If you additionally examine your mucus or cervix – which is easy to do – safety will increase up to 0.8 (details)
What is the disadvantage of Perimon? Perimon only designates max. twelve days per cycle as infertile and it needs ca. 3-8 cycles before it starts with a prediction. These are the limitations of the rhythm method that can be broadened yet by applying e.g. the cervical mucus observation (when required)