Ultrasound help

Im 18 +2 i had an appointment for the 7th of july

Because they want it closer to 19-20 weeks but i changed it to the 3rd, which i will be exactly 19 weeks, but heres the catch

For them im about 4 days less, but i know its wrong. Anyways they made a fit with me and gave ot for the 7th cus its closer to 19-20 weeks mark but im closer, and this appointment is to find out babys sex, their date or mines we land on the same mark. People fond out earlier so whats the big deal?