tracking modifications

Hello all!

I have a SharePoint list that has a bunch of columns.  I would like to track any time that most of these columns change.  For now, I just need a new column that shows the date and time that a specific (or a group of) column changes.

I understand that this is going to be a 'calculated' column, but I am not 100 % sure of the syntax of the code.  So far, my code is:  

=IF([column to watch]=Modified,TODAY(),"")

The system liked that, but it is not showing anything other than today's date and 12:00 for the time.  where am I going wrong?

thank you all!

David