Changing "Today" referance in a formula to a cell referance

With the help of other forum members I have managed to arrive at the formulas to do the aging of invoices. Attached is the sample workbook.

Now I am trying to change the today formula in the 30+ days etc to read from the Date cell. The reason I want to do this so that when I save the individual statement and I reopen it again, it does not calculate everything again with the “TODAY” date.

I have tried doing this by removing today and replacing it with the cell number “G4” but it is giving me an error all the time. How can this be done?