Understanding Ovulation!!

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So I am using two apps to tack everything and both apps are showing me different fertility days! This month I have decided to try using an ovulation test. After reading the clear blue advanced digital instructions...its telling me to start testing on a date even sooner. I'm so confused. 😟 Can anyone help with this? My cycle is 28 days. I started my period on 6/14 and its suppost to end today.