Due date question!

Ashley • 08-11-15 ❤️👶🏼CRJ
So I went to the doctor at 9.5 weeks and they said I was measuring exactly that (go Glow!), but then my due date according to them is 8/7/15 which would make me 18 weeks now instead of 17. 
I'm wondering if I should update the app to say my due date and go off that, or keep what I've got. I've only had one ultrasound, so I have no idea.