Ovulation & Period Calculation

Ok so, I don't know what I am doing wrong, but the app is incorrectly calculating my period due date. Following my last period I have worked out my correct next due date. I then worked out 14 days earlier for when I should be ovulating, but the app says ovulating a week earlier (this week). I've just done Ov test and I'm not ovulating - but that's no surprise now as I have calculated that it is actually next week? Maybe I should go back to old fashioned diary...