I manage to create 2 variables by using Opened At column as a reference to Mark's leaving date:
Variable 1
=If ([Opened At] > ToDate ("31/03/2013"; "dd/MM/yyyy");
If (Trim([Incident Assignee]) InList ("A";"B;"C");"Joe";
If (Trim([Incident Assignee]) InList ("D";"E;"F");"Mary";
If (Trim([Incident Assignee]) InList ("G";"H;"I");"Peter"; "Others"))))
Variable 2
=If ([Opened At] < ToDate ("01/04/2013"; "dd/MM/yyyy");
If (Trim([Incident Assignee]) InList ("A";"B;"C");"Mark";
If (Trim([Incident Assignee]) InList ("D";"E;"F");"Mary";
If (Trim([Incident Assignee]) InList ("G";"H;"I");"Peter"; "Others"))))
Then I created a new column in the report by adding this formula:
=Concatenation([Variable1];[Variable 2])
It worked.