if you have a 28 day cycle you should have ovulated @ the 17th, but if you don't then your first u/s should show the size of the baby and all you do is subtract 2 weeks and then that will show you your estimated date of conception! some people have longer cycles, and sperm can stay in the system i've heard for 7 days ( very unlikly that long )

if you assume a typical cycle of 28 days then your likely conception is the 17th.

sperm typically do not survive past 3-4 days.
I always stipulate 5 because I fell pregnant 5 days post sex (dedicated charter with super regular periods and ultrasound confirmed).
5 really is a long time for sperm to survive though.