How to freeze an excel calculation on a certain day

Background: At my place of work, we receive referrals for women who are pregnant. We have a master-list spreadsheet of all referrals we have ever had, and a separate weekly spreadsheet that only has women who were referred that week, or were referred recently, but we are still working on their case. 

I have used the following formula to calculate gestational age (how many weeks & days pregnant the women are) from their estimated date of delivery- 

=DOLLARFR((TODAY()-F3+280)/7,7) 

Where F3 is the cell with the estimated date of delivery

Question: For auditing purposes, we would like for the gestational age on the spreadsheet to reflect the gestational age at referral, and not continue to increase as days go by. Is there a way for me to freeze the calculation for the day I input it?

Please note I am very much a beginner and would appreciate any answers in simple terms.

Thank you in advance,

Lora