Merge rows excel problem

[#1]

Hi,

I have an excel table that looks like this :

id	statut	date_start	date_end	duree
id1	demande ouverte	01/05/2010 10:02		-
id1	demande ouverte		02/06/2010 11:13	-
id1	qualification	02/05/2010 13:20		-
id2	qualif	02/05/2010		-
id2	fermee	01/05/2010 12:00	03/05/2010 12:25	25

and I want to merge cells to obtain duree (duree= date_end-date_start)

I have already tryied with interval function

You can read code with attached file

Yours faithfully