Question: In SharePoint foundation 2010 can i add a new calculated value column where the calculation are based on when another column is modified ?

Answer: 
Out of the box, SharePoint won't tell you when a specific column was modified. You only know when the item was last modified (Modified field).

To track changes on a specific column, you would need to duplicate it and run a workflow that checks for updates. Not a simple design.

Here is how it would work:

* add two more columns hidden from the users: CopyColumn and CopyModified
* on item change, the workflow compares Column with CopyColumn
* if the values are different it means that the user has modified the column. Calculate the time difference between Modified and CopyModified, then update CopyModified and CopyColumn for the next round
* if the values are the same, no action, just stop the workflow
Yes, you can see this link for possible formulas - <http://msdn.microsoft.com/en-us/library/bb862071.aspx>