APP changes the due date for period?

This app said my period was due the 18th and when it didn't come I put in that it was late and the app changed it to be due the 20th. When it didn't come that day I put it was late and it changed it to be due the 22nd. My last period was March 21st and I've never been positive on how to tell how many days are in your cycle (I'm 24 I should know this by now 😐) so anyone know when the app will say if my period is actually late or is it just going to keep pushing the date back?