If My Due Date Is September 11th, What Time Did I Get Pregnant?

Your question is incomplete. You have not mentioned the first day of last menstrual period. Usually starting from first day of last period, add 40 weeks, this will be your expected date of delivery. If you deduct 2 weeks from expected date of delivery, it will be most probable conception date. But exact conception date can also be calculated from ultrasound by gestational age.

My due date is on September 11th- and I know for sure that I got pregnant on December 2nd. A full term pregnancy is 40 weeks, which make that closer to 10 months than 9.

Go on the Internet.  Use the search engine and put in due date calculator or something like that.  Usually that will tell you.
 
You can also ask your doctor.  He can tell you by one of those circle calender things they have to figure out how far along you are.  They just go by your due date.  And it says approx.  when you ovulated.
 
Also, if you know when the first day of your last period was.  You usually ovulate 7-10 days after the first day of your last period.  If you don't remember your doctor should have it on file.  He would have had to have an approx.  date to figure out how far along you are unless he did that by ultra sound.  Even then they can figure that out for you.
 
Good luck.

I'm due September 17, 2009 what day did I Conceive?

If my due date was dec 11 2009 what day did I get pregnant

If your due date is September 11th , conception would have been on December 19. Give or take, sometimes due dates are wrong. I calculate my ovulate days through this Ovulation Calculator, and I gain instant result

According to my humble opinion if your's due date is 11th September, then add 40 weeks for your pregnant date.

To track your pregnancy through your due date ,here are pregnancy week by week tracking by using pregnancy calculator.

Hope that helped!