Automatic date update in a cell when another cell's value changes (as calculated

Hi All,

I need an help one excel formula ,i have specific range with prices, but when ever there is a change in value in the range then one particular need to be updated with now() formula. Find below the snap shot of desired data....

Product Changes happened Jan'13 Feb'13 Mar'13 Apr'13 May'13 Jun'13 Jul'13 Sep'13
Apple
Orange
Pineapple
Fruits


In the above data i have values for all product in all months. if there is any change happened then in the changes happened column need to be updated with now() formula.

Thanks in advance...

Vinwin