I wanted to know how accurate early ultrasounds (first trimester) are in conception dating? If I was 5 weeks4days 9/23/13 putting conception 25-28 of aug could I have still concieved 7/28/13

Very accurate. Provided that the individual sono graphed is reliable.;the earlier sonograms are usuall more accurate.
No. An ultrasound at 5 weeks is extremely accurate. It is impossible for you to have conceived in july.