OpenHAB - DateTime format weather-warning

rule "DWD Warnungsdauer Kombi-Item"
when
  System started or
  Item DWD_Warnung_Start changed or
  Item DWD_Warnung_End changed
then
    var von = DateTime(DWD_Warnung_Start.state.toString)
    var bis = DateTime(DWD_Warnung_End.state.toString)
    DWD_Warnung_Dauer.postUpdate("Von: " + von.toString("dd.MM.yyyy HH:mm") + "Uhr Bis: " + bis.toString("dd.MM.yyyy HH:mm") + " Uhr")
    logInfo("DWD_Dauer", "Test: " + DWD_Warnung_Dauer.state)
end