DateDiff only allows you to choose a single unit.
The following function calculates the difference in minutes, then converts
that value to hh:mm
Function TimeDiff(Time1, Time2) As String
Dim lngMinutes As Long
lngMinutes = DateDiff("n", Time1, Time2)
TimeDiff = Format(lngMinutes \ 60, "0") & ":" & _
Format(lngMinutes Mod 60, "00")
End Function