Need help on due date thing

So when I test it's Bfp and we soo happy and can't wait to tell family and a few close friends. I test 3 days before my period suppose to start, it faint line. Then I test again 4 days late period, the lines show really clear. HERE MY QUESTION. How do I figure out caulatate my due date ? I Won't Able To Get To My Dr IN JAn. I download pregnancy one of the app. It's asked when is "date of your last period began"? So do I enter the one I just missed or the one before?

Sorry for long message, anyone can explain to me I'll appreciated it. My first time to be a mom.