biber: year = {28. September 2012} resultiert in Warnung "year field … is not an integer"

\begin{filecontents}{\jobname.bib}
    @Online{EUKOMa2012,
      Title                    = {Kapitel des Besitzstandes/Verhandlungskapitel},
      Author                   = {{Europäische Kommission}},
      Gender                   = {sf},
      Keywords                 = {primary, Kopenhagener-Kriterien, Beitritt, Beitrittsverhandlungen, Beitrittskriterien, cited},
      Shortauthor              = {Kommission},
      URL                      = {http://ec.europa.eu/enlargement/policy/glossary/terms/chapters_de.htm},
      Urldate                  = {2015-11-05},
      Year                     = {28. September 2012},
      Timestamp                = {2016.02.04}
    }
\end{filecontents}

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\begin{document}
\cite[Siehe][15]{EUKOMa2012}

\printbibliography
\end{document}