Edd 6th of July.

I'm so confused, I tested on the 23rd and got a very positive test and have since had strong ones. According to this app I am 3 weeks and 6 days, then according to another app I am 5 weeks and 6 days. When do you start counting the weeks? My period was on the 19th of September. I am seeing other people's tests and they are a lot lighter.