Kodu aşağıdaki şekilde değiştirerek dener misiniz...

Function Hesapla(BT As String, ST As String) As Double

Dim BT1, BT2, ST1, ST2 As Integer

Dim F1, F2, F3 As Integer

BT1 = CInt(Left(BT, 2))

BT2 = CInt(Mid(BT, 4, 2))

ST1 = CInt(Left(ST, 2))

ST2 = CInt(Mid(ST, 4, 2))

B1 = BT1 * 60 + BT2

S1 = ST1 * 60 + ST2

F1 = S1 - B1



F2 = Int(F1 / 60)

F3 = F1 - F2 * 60

If F1 <= 0 Then

Hesapla = (F2 + F3 / 60) + 24

Else

Hesapla = F2 + F3 / 60

End If

End Function