Insert a static version of today's date that doesn't update itself

Hi,
I need to insert a static version today's date without using the today() function. This function updates itself.
Basically, I have a spreadsheet that has a check box (made from the forms toolbar), that is linked to cell H11. Cell H7 has the following formula inside it

=IF(H11=TRUE,TODAY(),"")

This is great, because when the box is checked it inserts today's date into H8, and if the tickbox isn't checked H7 remains blank. However, if I open this document on another day it will update the date to the current date.
I need the formula to put in a non-updating date, e.g. the date that it was clicked. How can I do this?
Pete