<?
$date = date_create_from_format("D, d M Y H:i:s O", "Tue, 20 Mar 2018 01:38:22 GMT");
echo date_format($date, 'Y-m-d');

//result 2018-03-20

$date = date_create_from_format("D, d M Y H:i:s O", "Tue, 21 Mar 2018 01:38:22 GMT");
echo date_format($date, 'Y-m-d');

//result 2018-03-27  почему 27 марта?