When is conception date?

How do I calculate the pregnancy if I had sex the day BEFORE my period was supposed to come. If I go by my last normal period it says my conception day is May 20 which is way off. I didn't have sex till about 2 weeks later. Do I still use the first day of my last period or can I modify it till around the day I had sex. I'm sure it's that day, I've only had sex 1 time within 6 months! Please don't be rude, I'm not some careless teen, I'm married, have a job, etc.