What do YOU think my true due date is?

I have asked this question to just about everyone I know, and no one seems to be able to tell me a straightforward answer. Here's why: I had my last period Dec 4, 2008 which would make my due date Sept 10, 2009. My first ultrasound showed the baby a little smaller than average, so the U/S tech pushed my due date out to the 24th of Sept. BUT, I got a very clear positive reading on a dollar store test on January 21, 2009. IF my due date is in fact the 24th, would I have gotten such a strong reading in January? This has been a question that has rolled around in my head since the moment the U/S tech changed my due date. Just curious to hear what others think. Thanks, in advance!