calculating dates OBGYN office needs help

[BetseyRod]

I work in an OBGYN office and we have to calculate how far along a patient is based on information we enter from an ultrasound done at her 1st appt. The patient returns back 3 days later - how can get it to update how far along she is now  

8/31/2021 she is 10 weeks 5 days returns 9/3/2021 she is now 11 weeks 1 day

help please

[#2]

@BetseyRod Easiest shown in a working example. Note I added a few helper cells that calculate Days. From there it's easy to calculate whole weeks and remaining days. See attached.