stopping excel from updating formula after a crite

Question

Hi i need help with stopping excel from updating formula after a criteria is matched. I have the following:

Cell A1 is =TODAY() Cell

B1 is Date Received

Cell C1 is Number of days open.(contains the formula) This obtained by the following formula:

=IF(B1="","-",IF(NETWORKDAYS($B1,$A$1)>0,NETWORKDAYS($C1,$A$1),"-"))

This returns the number of days a sales quote has been open.

The second part is once the invoice has been done and now I have a second date to use.

Cell D1 is Date the invoice is completed Cell E1 is number of days taken to invoice(contains the formula)

This is obtained by the following formula:

=IF(NETWORKDAYS($B1,$D1)>0, NETWORKDAYS($B1,$D1),"-")

What I need is once Cell D1 is filled out that Excell freezes Cell C1 and not continue counting days between B1 and A1 as cell A1 will update each new day? Can this be done and how if so?