I changed the configuration
of Mantis to have the Dates german style:

$g_short_date_format = 'd.m.y';
$g_normal_date_format = 'd.m.y H:i';
$g_complete_date_format = 'd.m.Y H:i T';
$g_calendar_js_date_format = '\%d.\%m.\%y \%H:\%M';
$g_calendar_date_format = 'd.m.y H:i';