I just downloaded the <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> and had previously been using my period calender. The fertile days on each app are different and the period start dates were different. Old said I should start Thursday and glow said Monday. I got AF today :( so I guess glow is more accurate. So I should follow the fertile days on glow instead, maybe that's what I did wrong with the other app. I just always thought you ovulated 14 days after you start your period.