found by adding
 Date& operator+=(long) = delete;
to Date class

Change-Id: I0e6b59a2e789f2bdf2f271b99d10c6acbae8bf55
Reviewed-on: https://gerrit.libreoffice.org/40241Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>