Function IstSommerzeit(zeit As Date, zone As TimeZoneInfo) As Boolean
    Return zone.IsDaylightSavingTime(zeit)
  End Function

' in Listbox eintragen:
ListBox1.Items.AddRange(TimeZoneInfo.GetSystemTimeZones.ToArray)
ListBox1.SelectedItem = TimeZoneInfo.Local
 
Private Sub ListBox1_SelectedIndexChanged(sender As Object, e As EventArgs) _
  Handles ListBox1.SelectedIndexChanged
  Dim zi As TimeZoneInfo = CType(ComboBox1.SelectedItem, TimeZoneInfo)
  Dim sz As Boolean = IstSommerzeit(Date.UtcNow, zi)
End Sub