sort.date as function for bottoming out invalid values


git-svn-id: svn://oss.dbc.dk/sync_php/php/OpenLibrary/OpenBibdk/trunk@68816 30e9dc4a-7611-4fbc-83ba-eecf79b76bd0
git-svn-id: svn://oss.dbc.dk/sync_php/php/OpenLibrary/OpenBibdk/trunk@68818 30e9dc4a-7611-4fbc-83ba-eecf79b76bd0