<?php
  date_default_timezone_set("Europe/London");
  $ts = new DateTime("2018-03-17 15:53:55");

  $ts->modify("+2 days");

  $ts->modify("Friday +3 weeks");

  $ts->modify("next friday");
?>
Result:

Mon, 17 Mar 2018 15:53:55 +0000

Mon, 19 Mar 2018 15:53:55 +0000

Fri, 13 Apr 2018 00:00:00 +0100

Fri, 20 Apr 2018 00:00:00 +0100