Which period/ovulation tracker app is best?

Celeste • Pregnant with baby #1 EDD April 3rd, 2016!
I have 7 different apps and almost all of them tell me my period is going to start on a different date! How do I know when I am late? Any experience with any of these? 
period tracker: yesterday
Ovia: today
Period diary: today
Period log: today
Monthly Cycles: tomorrow
Glow: 3 more days