This may give you an idea about how to go about it:
' a Macro to calculate the elapsed time for formfields with Date format of
HH:mm
'
Start = ActiveDocument.FormFields("Text1").Result
StartHour = Val(Left(Start, 2))
StartMinutes = Val(Right(Start, 2))
StartTimeMinutes = StartHour * 60 + StartMinutes
Finish = ActiveDocument.FormFields("Text2").Result
FinishHour = Val(Left(Finish, 2))
FinishMinutes = Val(Right(Finish, 2))
FinishTimeMinutes = FinishHour * 60 + FinishMinutes
ElapsedMinutes = FinishTimeMinutes - StartTimeMinutes
ElapsedHours = Int(ElapsedMinutes / 60)
ElapsedMinutes = ElapsedMinutes - ElapsedHours * 60
ActiveDocument.FormFields("Text3").Result = Str(ElapsedHours) & ":" &
Format(ElapsedMinutes, "00")
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Hope this helps,
Doug Robbins - Word MVP