PHP：使用日期函数转换为日期后再次更改时间戳的原因是什么？

I am getting an input in milliseconds. E.G: $timestamp = 1479844722190;

I convert that timestamp using $date = date('Y-m-d h:i:s', $timestamp / 1000);

I convert back to a timestamp using strtotime($date);

My result is 1479801522

The same thing happens when using gmdate().

The actual code:

date():

    $timestamp = get_timestamp(); // 1479847575022
    $date = date('Y-m-d h:i:s', $timestamp / 1000); // 2016-11-22 08:46:15
    strtotime($date); // 1479804375

gmdate():

    $timestamp = get_timestamp(); // 1479847661576
    $date = gmdate('Y-m-d h:i:s', $timestamp / 1000); // 2016-11-22 08:47:41
    strtotime($date); // 1479804461

Could someone tell my why this happens, and whether there is a way to do this without creating a time-machine like this?

UPDATE:

Putting date_default_timezone_set('UTC') beforehand gives the following results:

timestamp: 1479848532028
date: 2016-11-22 09:02:12
reverted: 1479805332

timestamp: 1479848622222
date: 2016-11-22 09:03:42
reverted: 1479805422

After a sleeping on it, I found the answer.

gmdate('Y-m-d h:i:s', $timestamp / 1000);

should really be:

gmdate('Y-m-d H:i:s', $timestamp / 1000);