pregnancy test?

I have two apps that let me track my period, this one and period diary, because I'm paranoid about getting pregnant, i am normally very regular and get my period the same day every month
now both apps say my period is late, by 13 and 2 days. I had unprotected sex with my boyfriend 2 weeks and a day ago, I've been extremely hungry, rarely tired but not nauseous at all. 
I'm probably going to take a pregnancy test within the next week if I don't get my period, when's the most accurate time I could take the test?