Option Explicit
Dim dteSumme As Date

Private Sub chkMin_Click()
  If chkMin.Value Then
     'Anzeige: (MM)MM:SS
    lblSumme.Caption = Format(dteSumme * 24 * 60, "00") & _
       Format(dteSumme, ":ss")
  Else
     'Anzeige: (HH)HH:MM:SS
    lblSumme.Caption = Format(Fix(dteSumme * 24), "00") & _
       Format(dteSumme, ":nn:ss")
  End If
End Sub

Private Sub cmdPlus_Click()
   'Zeit addieren
  dteSumme = dteSumme + TimeSerial(CInt(txtStunden.Text), _
    CInt(txtMinuten.Text), CInt(txtSekunden.Text))
  If chkMin.Value Then
     'Anzeige: (MM)MM:SS
    lblSumme.Caption = Format(dteSumme * 24 * 60, "00") & _
       Format(dteSumme, ":ss")
  Else
     'Anzeige: (HH)HH:MM:SS
    lblSumme.Caption = Format(Fix(dteSumme * 24), "00") & _
       Format(dteSumme, ":nn:ss")
  End If
End Sub

Private Sub cmdMinus_Click()
   'Zeit subtrahieren
  dteSumme = dteSumme - TimeSerial(CInt(txtStunden.Text), _
     CInt(txtMinuten.Text), CInt(txtSekunden.Text))
  If dteSumme < 0 Then dteSumme = 0 'Es gibt keine negative Zeit!
  If chkMin.Value Then
     'Anzeige: (MM)MM:SS
    lblSumme.Caption = Format(dteSumme * 24 * 60, "00") & _
       Format(dteSumme, ":ss")
  Else
     'Anzeige: (HH)HH:MM:SS
    lblSumme.Caption = Format(Fix(dteSumme * 24), "00") & _
       Format(dteSumme, ":nn:ss")
  End If

End Sub

Private Sub cmdDelAll_Click()
   'Alles löschen
  txtStunden.Text  = "00"
  txtMinuten.Text  = "00"
  txtSekunden.Text = "00"

  If chkMin.Value Then
    lblSumme.Caption = "00:00"
  Else
    lblSumme.Caption = "00:00:00"
  End If

  dteSumme = 0

End Sub

Private Sub cmdBeenden_Click()
  Unload Me
  End
End Sub

Private Sub Form_Unload(Cancel As Integer)
  Set frmTimeCalc = Nothing
End Sub

 'Bei GotFocus gesamten Text in Box markieren
Private Sub txtStunden_GotFocus()
  txtStunden.SelStart = 0
  txtStunden.SelLength = Len(txtStunden.Text)
End Sub

Private Sub txtMinuten_GotFocus()
  txtMinuten.SelStart = 0
  txtMinuten.SelLength = Len(txtMinuten.Text)
End Sub

Private Sub txtSekunden_GotFocus()
  txtSekunden.SelStart = 0
  txtSekunden.SelLength = Len(txtSekunden.Text)
End Sub