How to tell how far along u are? Really confusing

So I got my bfp 10 days ago, so I downloaded 2 different apps on my tablet and put in my last day of my period which was Feb. 3 and the app is saying I conceived on the 17th and am now 6 weeks pregnant. I don't understand how that can be? From the 17th of Feb to March 18 is not 6 weeks. I don't get this pregnancy stuff.