Description:
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Adding  7 days to the timestamp for 2011-11-05 increased the date by 6 days only. run sample code. 

 

Test script:
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/*
It appears that the day function of the time stamp is falling behind . 
*/ 
 	$ts = mktime(0,0,0,10,29,2011) + 7*86400 ;
	$mday1 = date('d',$ts);  //OK

	$ts = mktime(0,0,0,10,29,2011) + 14*86400 ;
	$mday2 = date('d',$ts); //1 day behind
	
	$ts = mktime(0,0,0,10,29,2011) + 21*86400 ;
	$mday3 = date('d',$ts); // 1 day behind

echo $mday1. ' ' . $mday2.' '. $mday3 ;