If my obgyn gave me a due date of September 23rd by ultrasound. My lmp was December 24th 2009. By the due date he gave me mean I got pregnant either on or 1 day after my period. I mean, it doesn't make sense. Could he be off by a week or so? He says that we did a early u/s and is pretty confident in the dates.

Dates from an ultrasound are an "educated guess". They can be off by a week or more. Its also possible that you ovulated sooner than you expected.

He's guessing. At best, dating from an ultrasound is an educated guess, and if he's within a couple of weeks of your date, he's not doing too badly. But go by your dates, particularly if you are fairly sure about when you conceived.

On my first baby, I knew exactly the date and time I conceived, because we'd been on holiday with a group and had only had privacy for sex once in the entire three weeks we were away. But that didn't stop the staff at the hospital giving us all sorts of dates we knew were wrong.

All due dates are guesses, ( educated of course) so give or take a couple weeks is where it is at.