Ovulation question

So are you supposed to have sex before or after you ovulate to get pregnant? Also I'm using two <a href="https://play.google.com/store/apps/details?id=com.glow.android">fertility apps</a> one being . Diary that says I am supposed to ovulate on the eighth and that I'm fertile from December 1 to December 11 and I'm also using Glow which says that I'm supposed to be fertile from the eighth until the 12th I'm just confused as to which app is more accurate and which one I should follow?