Changing start date

Julea
My period was suppose to start on Saturday August 1st. But it didn't. I took a test on the 3rd and it was negative.. And now this app has rescheduled my period to start Aug 10.  I don't understand how it came up with this date? It still hasn't came so maybe the app is right and I will start on the 10th. I'm just wondering how it's figuring that date??