That's what I have always heard too. There are only 24 hours out of a whole month that a woman can get pregnant. I worked it out somehow when I was trying for my second child but I can't remember exactly what I did. It worked though.
Try this link http://www.4woman.gov/Pregnancy/ovulation1.cfm It's supposed to help you figure out when you are ovulating so you can get pregnant, you can use it the opposite way LOL