[date_metadata_update & date_creation] and for these fields the format_date
function is called with a 'mysql_datetime' format value (so, the function is
waiting for 'yyyy-mm-dd hh:mm:ss' date format)
Made a call with the 'us' to resolve the problem
git-svn-id: http://piwigo.org/svn/branches/2.0@2850 68402e56-0260-453c-a942-63ccdbb3a9ee