How do I calculate a formula once
 

Can anyone help?
I have a table of values and dates. I want to calculate a formula when a
date on another sheet matches a date in the table. I have used an if
statement to do this but when the date changes the IF statement recalculates
and sets the values in teh table to FALSE. I want to retain the value in the
table and only update when the dates match to provide a "history".

How can I do this? This is the IF formula
=IF(A4=Header!$B$9,(SUM($D$2:$D$34)/PLine!$D$38))
A4 is the date in the table.

Thanks