How do you know how far along you are

kathy
I'm trying to figure out how far along I am. First day of my last AF was Sept 19 and last day was sept 23. Glow says I'm 4 weeks and 5 days. That seems like it's based on the last day of the period but everything else I've read is based on the first day. I'm confused. What is correct?
When you go to the doctor, is that when you get the "real answer"?
And what do doctors base it on (best guesstimate of the day when the sperm entered the egg?)