Fertile days? Help!

Ok so for about almost two years I've used this app called simple period. It's always worked lol I guess because so far I haven't gotten pregnant . My question is that app says my fertile days are from let's say this month 16-20 which are 14 days after my first period day . But this one says from the 12-18th and even though it's around the same area I still don't know which to follow. I don't want to end up pregnant anytime soon. Lol can you ladies help me? Has this app really helped y'all ? Either trying to conceive or not trying? Just want to see the accuracy .