[AccessD] Problem: subtotaling time (long email)

Dear List

I cannot figure out how to add time.

I have a database that tracks responses for emergency services providers.

Table and report details are below. I'm trying to create a report showing
each provider's responses, sorted and subtotaled by major class types and
then minor class types, with the time of each incident shown AND subtotaled.
I have no problem with the report EXCEPT that I cannot subtotal the time
(the "Time = " and "Total Time = " fields).

I suspect that the problem may be that I am using a formatted field
(TimeDiff), and cannot add the data because they are now string values. If
anyone can help steer me in a better direction, I would be extremely
grateful!

TIA,

Bob Gajewski

*************************************************************
DATABASE DETAILS:

tblEmployees
	EmployeeID (PK)
	EmployeeName

tblMajorClass
	MajorClassID (PK)
	MajorClassName

tblMinorClass
	MinorClassID (PK)
	MinorClassName

tblIncidents
	IncidentID (PK)
	MajorClassID (FK)
		(has "Call" or "Drill")
	MinorClassID (FK)
		(for "Call", has "Fire" or "EMS")
		(for "Drill", has "Internal" or "External")
	DispatchTime
	InServiceTime

tblResponses
	ResponseID (PK)
	IncidentID (FK)
	EmployeeID (FK)

The report (rptEmployeeResponse) has the following:

	Sorting & Grouping:
		EmployeeName (Ascending)
			Group Header = Yes
			Group Footer = Yes
			Group On = Each Value
		MajorClassName (Ascending)
			Group Header = Yes
			Group Footer = Yes
			Group On = Each Value
		MinorClassName (Ascending)
			Group Header = Yes
			Group Footer = Yes
			Group On = Each Value
		IncidentID
			Group Header = No
			Group Footer = No
			Group On = Each Value

	SQL Statement (QBE)
		EmployeeName (GroupBy)
		MajorClass (GroupBy)
		MinorClass (GroupBy)
		IncidentID (Count) - from tblResponses!
		DispatchTime (GroupBy)
		InServiceTime (GroupBy)
		TimeDiff (GroupBy)
			TimeDiff:
IIf([InServiceTime]>[DispatchTime],Format([InServiceTime]-[DispatchTime]),"S
hort Time"),Format(DateAdd("d",1,[InServiceTime])-[DispatchTime]),"Short
Time"))

	Sample Output:


Brown, Joe
	Calls
		EMS
			001	11:00	13:59	2:59
			003	09:13	10:29	1:16
		Count = 2		Time = 4:15

		Fire
			002	23:04	01:41	2:37
			004	11:42	12:02	0:20
			005	04:30	05:58	1:28
		Count = 3		Time = 4:25

	Total Count = 5	Total Time = 8:40


	Drills
		External
			001	11:00	12:00	1:00
			003	09:00	10:30	1:30
		Count = 2		Time = 2:30

		EMS
			002	19:00	21:30	2:30
			004	11:00	12:00	1:00
			005	09:30	11:00	1:30
		Count = 3		Time = 5:00

	Total Count = 5	Total Time = 7:30

{Next Employee ....}

Counts and times are only totaled per employee; there is no aggregate grand
total.