Her last menstrual started either 8/19 or 8/20 and ended 8/25. It came a week early. She was with me 8/29 & 8/30 and her ex 9/1. I gave her 2 pregnancy test on the 9/21 and the both said she pregnant. Who is most likely the father? She tried for 3yrs to get by pregnant by him and only got pregnant once and had a miscarriage. We've been having unprotected sex for 6 months now and she's pregnant

There's no way to know until a DNA test is done. The dates are too close together.

You can determine conception dates down to give or take a week or two weeks.

There is no way at all to determine conception within a day. You will have to wait till child is born and do a DNA test.