Due date discrepancy?

Hello all! I am pregnant with my second baby. the first day of my last. Was the 17th of March. That would put my due date on the 22nd of December. I had my first ultrasound appointment this past week, and the baby is measuring 1 week and 3 days earlier. The doctor said unless the baby is measuring behind or ahead more than 2 weeks, they usually keep the initial due date. However, for this app, should I change the due date so the baby progress matches the measurement of the baby or keep it according to my last period? If I keep it to my last period, I'm 9weeks and 1day. If I change it to the baby's measurements, I am 7weeks and 5days.

My transvaginal ultrasound of the yolk sack (on the left) and the baby (on the right).