Sum a calculated date/time field

When trying to sum the date time field I get a aggregate error that refers to
the first field in the query that has nothing to do with the calculated
field. I am able to convert it into integer format and subtract two date
fields to get the total time. But when I try to sum the total time I get the
aggregate error refering to the first field in query. This is the calculation
I use to get the field I would like to sum. It works great but can not figure
out how to sum it without the aggregate error.

Elapsed Time: Int(CSng(([actual completion time]-[actual start time])*24)) &
":" & Format([actual completion time]-[actual start time],"nn:ss")

Can anyone help me?
--
Lucy