Dim x As String
 
Private Sub Form_Load()
    x = 0
    Timer1.Interval = 1
    Timer1.Enabled = True
End Sub
 
Private Sub Timer1_Timer()
    If Len(x) > 3 Then
        x = Replace$(x, ":", "")
    End If
    x = x + 1
    If Len(x) > 3 Then
        x = Left(x, Len(x) - 3) & ":" & Right(x, 3)
    End If
    Label1.Caption = x
End Sub