With my son he was due 26th of march 2010 and I conceived him July 3rd, so it would probably be around July 18th +/- 5 days (based on a 28 day cycle) but this is only a rough guess. As kitkat said if this is a paternity matter only a DNA test can tell once the baby is born.
I am due April 19th with a 30 day cycle, I am positive I conceived on July 28 cause of light spotting
Go to Google and type in conception calender and that will tell you
There's no way to know exactly when you conceive as sperm can live inside for around 5 days and it can take around a week or so after ovulation for the egg to implant etc. This is why doctors use measurements etc when dating your pregnancy but you can't tell exactly when conception happens. If this is because of a paternity issue, then a DNA test when the baby is born is the only way to know for certain.
My pregnancy app says around July 19th