the app that helped me with finding out when was my last LMP was the called "I'm Pregnant " this app is really good as this one, you should try it :) but keep this app too lol i have both this one and the other one lol
just putting your due date will tell you the date and month of your last period, that'll be the day you probably convince. then you start counting the day it conceived and I'll lead to how many weeks you're :) like i conceived on a Wednesday 11/5/14 so starting the next week is when the weeks of my pregnancy start ,so every Wednesday i turn a week :) now i am 11+3 weeks hope this helped