Since you don't have a duration if one (or both) of the dates are Null, have
the function return 0 in that case.

Function Minus_Non_Work_Time(BegDate As Variant, EndDate As Variant) As
Integer
' Note that this function does not account for holidays.
' MODIFIED FROM CODE FOUND ON DEV ASHISH'S SITE
' ASSUMES A PROJECT CANNOT BE ENDED ON A WEEKEND
Dim WholeWeeks As Variant
Dim DateCnt As Variant
Dim EndDays As Integer
Dim oddweekend As Integer
Dim weekends As Integer
Dim Days As Integer

 If IsNull(BegDate) Or IsNull(EndDate) Then
       Minus_Non_Work_Time = 0
 Else
       BegDate = DateValue(BegDate)

       EndDate = DateValue(EndDate)
       ' compute the number of full 7 day weeks
       WholeWeeks = DateDiff("w", BegDate, EndDate)

       ' now add remaining days, skipping sat & sun
       DateCnt = DateAdd("ww", WholeWeeks, BegDate)
       EndDays = 0
       oddweekend = 0

       Do While DateCnt < EndDate
               If Format(DateCnt, "ddd") <> "Sun" And _
                         Format(DateCnt, "ddd") <> "Sat" Then
                       EndDays = EndDays + 1
               Else ' ADD 24 HRS FOR EACH WEEKEND DAY
                   oddweekend = oddweekend + 24
                   MsgBox DateCnt & "  " & EndDate

               End If
               DateCnt = DateAdd("d", 1, DateCnt)
       Loop
       ' number of working days

       Days = WholeWeeks * 5 + EndDays
       'compute non-working hours

       Minus_Non_Work_Time = (Days * 15) + (WholeWeeks * 48) + oddweekend
 End If

End Function