If you are using macros to create the unique reference, you could use macro to populate the cell with current datetime value and then use it as unique reference. If you do not want to use macros, there is a work around, but you must understand that this is an application level workaround, which means that it could affect other workbooks that are open.

The NOW function is volatile in nature. That's why it calculates everytime the file is saved or reopened. To overcome this issue, try this..

Enable Iteration in your application (this is the work around I was suggesting)

And use this formula in A1

=IF(A1=0,NOW(),A1)