Sort two WordPress posts by the value of a meta key, "event-date," assumed to be a string parseable by strtotime

add-to-functions.php

function sort_by_event_date($post1, $post2) {
  $date1 = strtotime(get_post_meta($post1->ID, 'event-date', true));
  $date2 = strtotime(get_post_meta($post2->ID, 'event-date', true));
  // I've reversed this now so that it sorts in descending order, newest first:
  return ($date1 == $date2) ? 0 : ($date1 > $date2 ? -1 : 1);
}