How can you be certain that everything this App tells you about your Fertile Window is accurate when this App is off on your expected menstrual each month?

I've been using this App for a year now and it's Never accurate on the start date for my menstrual cycle ( each month it gives an accurate start date for your menstrual cycle but it's always a week wrong ) according to this App I'm not suppose to start until next week but again like last month and the months before I'm starting a week before the date this App proclaims I'm suppose to start. So again How can we be certain that the date of our Fertile Window is accurate when this App is off with our menstrual cycles each month?