How many weeks?

Sara
According to this app, I'm 8 weeks pregnant and should be due on August 31st! I'm just not sure how accurate it is, and I'm driving myself a little crazy trying to figure it out. 
My last period was on November 25th, which would make the 40th week August 31st. However, my cycles were normally 37-39 days long instead of the normal 25ish. Does my longer cycle affect my due date at all? I go to the OB on Tuesday for my first appointment, but that still seems like it's so far away!!