If my Last menstral period is 11/23/06, and I have an expected due date of 8/29/07 can my conception date possibly be December 1st?

If you go to:- Pregnancy Dates Calculator (http://www.intmed.mcw.edu/clincalc/pregnancy.html) type in the first day of your last period, younwill get 7th Dec as your approximate conception date. But that makes your due date for 40 weeks 30th August.