<?php

date_default_timezone_set('America/Chicago');
header('content-type: text/plain'); 

$ts = mktime(0,0,0,10,29,2011) + 7*86400 ;
var_dump(date('c',$ts));  //OK

$ts = mktime(0,0,0,10,29,2011) + 14*86400 ;
var_dump(date('c',$ts)); //1 day behind
	
$ts = mktime(0,0,0,10,29,2011) + 21*86400 ;
var_dump(date('c',$ts)); // 1 day behind

string(25) "2011-11-05T00:00:00-05:00"
string(25) "2011-11-11T23:00:00-06:00"
string(25) "2011-11-18T23:00:00-06:00"