您可以尝试date() 功能:
$date = get_post_meta($post->ID, \'event_start_date\', true);
$day = date(\'d\', strtotime($date)); //01-31
另一种方式是
DateTime:
$date = get_post_meta($post->ID, \'event_start_date\', true);
$date = DateTime::createFromFormat(\'Y-m-d\', $date);
$day = $date->format(\'d\');