SortRepeaters.php

<?php

// Assuming you have a repeater field named `performances` and within each set there's a field named performancetime which is a datetime.
// The example is in PHP code, if you want to use it in Twig then you'll need to put this code in a method and use a Twig Filter to point
// to the method


$iterator = $record['performances']->getIterator();
$iterator->uasort(function ($first, $second) {
    if ($first === $second) {
        return 0;
    }
    return (float) $first->get('performancetime')->format("U.u") < (float) $second->get('performancetime')->format("U.u") ? -1 : 1;
});