At Planned Parenthood, they will use a conception calculation based on the first day of your last period. All of those calculators say "ESTIMATED" day of conception, not day of conception. They cannot predict that closely.
Get an ultrasound in your 7th week of pregnancy (counting from the first day of your last period) or your 8th week, and write back when you have an estimated due date that was actually based on the measurements of the embryo as seen in the ultrasound. Drop me a line in the DNA/Paternity forum and I will help you try to see if it is either of those dates that you listed. (Have you had sex any other times recently?)
Sperm can live up to 5 days in your system. So it can be either Guy. You are going to need a DNA test when the baby is born.
I was told that at planned parent hood that's when my baby was conceived.
No, not necessarily. Sperm can live five or six days in your system, so both guys' sperm would be present in your system at the same time. How do you know you conceived on the 15th, were you taking ovulation tests?