Wrong due date

So I have two different apps giving me 2 different due dates & come to find out, this app have the wrong due date..the other app says I'm 16wks & 3 days, this app says 15wks & 5 days but according to my doc she said I'm due between Aug 1st - Aug 12th..she didn't give me a specific due date but I calculated it myself & came up with Aug 9th so that's what I'm sticking to...btw do anybody else have this problem?