Moving my due date down?

Kailey

So just had an ultrasound done today and he is measuring a week ahead of my due date? He measured right where he was suppose to be at the 20 weeks scan. Now I'm suppose to be 24 weeks 1 day but he's at 24 wks 5 days. I was due on June 21st and now it's the 17th? Am I still suppose to go off my first due date or the new US?