Does anyone use Period tracker also?

Na
I was using <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">period tracker</a> for over a year to find out when I'm ovulating and I found this app today; <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">period tracker</a> says I'll get my period on the 15th, Glow says the 10th.  Seems like a big difference to me :/

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