Is this possible: whenever the value of a cell changes that value and the time of the change is recorded?

For instance, if cell A1 begins with a value of 5, then changes to 4, that new value is recorded:

(4, 10/26/2011 1:42am)
...
and so forth.

The changes would be as the result of a DDE link; not a user typing into a cell. Thanks for any help!

Greg