relative_time.php

<?php
/**
 * Short description for relative_time.php
 *
 * @package relative_time
 * @author Kazuki Hashimoto <eru.tndl@gmail.com>
 * @version 0.1
 * @copyright (C) 2016 Kazuki Hashimoto <eru.tndl@gmail.com>
 * @license MIT
 */

$now = time();
$youtube_timestamp = 1475471853;
$youtube_timestamp -= 86400 * 31;

$diff = $now - $youtube_timestamp;

// 6days = 518400sec
if ($diff <= 518400) {
    $time_text = floor($diff / 86400) . '日前';
}
// 13days = 1123200sec
elseif ($diff <= 1123200) {
    $time_text = '1週前';
}
// 20days = 1728000sec
elseif ($diff <= 1728000) {
    $time_text = '2週前';
}
// 27days = 2332800sec
elseif ($diff <= 2332800) {
    $time_text = '3週前';
}
// 30days = 2592000sec
elseif ($diff <= 2592000) {
    $time_text = '1ヶ月前';
}
else {
    $time_text = date('m月d日', $youtube_timestamp);
}

echo $time_text;