While the information in the "Knowledge" section is great, the calculator itself is terribly problematic. It assumes everyone's luteal phase is sixteen days (mine tends to be 11-13), and every time I entered a period, it would back up my ovulation date that same cycle to 16 days before the period started, ignoring my temp readings and OPK's! So, I'm so glad I continued to keep track on paper, so I could look back and know when I'd ACTUALLY ovulated each month (which was wa-a-ay off what the app said). And, twice in about three years, I had cycles that were about 17 days, and it told me that this was not possible. Thanks a lot. Yeah, it's possible, it happened to me. It should at least ask if I was sure, and then maybe, if it happened repeatedly, urge me to talk with my doctor, rather than just tell me that's impossible. Boo.