Returns an ISO8601 formatted date based on the given date.
Callback for use within hook_rdf_mapping() implementations.
$date: A UNIX timestamp.
string An ISO8601 formatted date.
function date_iso8601($date) {
// The DATE_ISO8601 constant cannot be used here because it does not match
// date('c') and produces invalid RDF markup.
return date('c', $date);
}