How To Determine The Exact Day Of Conception?

Hi doc, my due date was on feb.29 and had my baby on Feb. 21st (40 week mark) if I go back to the 40 weeks it takes me to May 25th the first day of the start of my 40 weeks. My lmp was on May 2-15 I really don't know, but how is it that it gives me May 25th as the first day of the start of my 40 week? I had sexual intercourse on May 24th..