Date Diff in Webi

Hello,

i have two columns in a MS-SQL DB which shows a login date and a logout date (both datetime2 datatype).

Login Date: 2014-10-15 09:29:12.772

Logout Date: 2014-10-15 09:31:32.318

Now i want to have the difference between this two dates in seconds, minutes or hours. i played with in the information design tool but no chance(it's a multisource universe). I tested with timestampdiff Is it possible to get the difference in web bi with a variable?

The Data Type of the Login / Logout Dimension in the Universe is DateTime.

I use Webi 4.0.