Excel: Last edit date of a range of cells?

I want one cell to show the date that a range of cells was last edited, and not update itself otherwise.

Example:

1) A1:D4 were last edited on 5/18/2008, and cell E1 displays that date
2) I edit cell B3
3) Cell E1 now shows 6/25/2008
4) I open the worksheet a week later, and cell E1 still displays 6/25/2008

etc, etc.

This is for a budget sheet, the use being that when I check my bank account balance online, I will know not to add any transaction amounts from before the last edit date to the cells that were already updated, eliminating the chance of erroneously counting a transaction twice.