How to write such a function in smarty?

Question

Smarty

How to write such a function in smarty?

usort($datings, function($a, $b) {
$ad = strtotime($a['date']);
$bd = strtotime($b['date']);
if ($ad == $bd) {
return 0;
}
return $ad < $bd ? -1 : 1;
});