There is a way, fraught with danger. If you enable circular references (Options\Calculation and check "Iteration") you can write formulas that self-reference. Enable CR's, clear the contents of cell N5, and enter the following formula in cell O5:
Code:
=IF(NOT(O5=0),O5,IF(ISBLANK(N5),0,TODAY()))
(I've assumed that column O is where you want the date.) Once this is in cell O5, copy it down as far as you'll need, plus a few rows.
1. The formula first looks to it's own cell to find out if there's a date there already; if so the formula puts that date back in the cell. (For this reason, if you need to extend this formula down to more rows later, you must copy it from a cell whose result is zero, not a date.)
2. Then the formula looks at cell N5; if it's NOT empty, the formula puts today's date in the cell.
You'll now have zeroes in those cells until you enter something in the corresponding cells in column N. (Anything: text, numbers, formulas, they all trigger the ISBLANK logic in column O.)
The danger is you are now unprotected from ACCIDENTAL circular references in other cells. If that's not a concern, the above should work.