I'm soo confused about some pregnancy app's on the Google play store some of them ask ur the first…

I'm soo confused about some pregnancy app's on the Google play store some of them ask ur the first day of ur last period n then some of them ask the last day of ur period so my first day of my last period was Nov14th soo dnt tht make me 5 weeks pregnant or 4 weeks it's so confusing... Could anybody help me with this lol I kno it's silly 😄