Sharepoint 2010 Calculated Column: How to get value before item gets modified?

I have a list that has a Status-field. If the status changes, a modified date in another column should indicate, when the status has been changed.

My idea was to create a duplicate of this Status-Column and create in a column, where the date is supposed to be, a function something like this:

=IF(NOT((Status)=(StatusDuplicate)),(Modified),(Stay like that(not specified yet)))

However, this (of course) doesn’t work properly. The Modified-Date-field gets updated even if I just “edit” the item with no changes.

Does someone know a solution/workaround to solve this problem?