using the difference between date in sharepoint

Dear Sir

i have a simple question
i create a workflow in sharepoint server and i want to return the difference in number of days between the due date to a following task and the current date.
i create a new column name time left.
so if the due date of a task is 15-3-2011
and the current date is 12-3-2011
i want the time left column to return 3
(15-12 =3)
i think it is a simple question but i can't figure how do it

Regards