DATEDIFF

I created a forum topic: http://drupal.org/node/578892 Not sure where i should post to get help!

I need to calculate the duration from the node has been created to last comment and node created and now.
Here are my aproached using the views calc calculation fields:
DATEDIFF(day,%node.created,%node_comment_statistics.last_comment_timestamp)

and

DATEDIFF(day,%node.created,NOW())

I added the following values in "Allowable functions and operators: "
+
-
*
/
(
)
,
'
CONCAT
MIN
MAX
ROUND
NOW()
DATEDIFF
day

But still no luck please help me!