Trust issues

What am I supposed to do if the father of my baby dosnt think he's the father because the timing of things is just two seconds off from being too late? He isn't understanding that ovulation fertility lasts 7 days from time of shedding the egg. Which happens 14 days before the period. I got pregnant on the 20th, my last period was the 27th of the month before. Sure numbers fall close, but the baby is still his... I just dont know what to do, how am I supposed to feel? I can't prove it to him...this is one of those moments where I wish we could replay memories.