<?
        //디데이 구하는 계산식 추가 - 20151127 gdkwon
        $days1 = intval((strtotime(date("Y-m-d",time())) - strtotime("2016-05-28")) / 86400);

        if($days1>0){
                $dmark = "+";
                $days2 = $days1;
        }else{
                $dmark = substr($days1,0,1);    //$days1의 부호
                $days2 = substr($days1,1);
        }

        $dslen = strlen($days2);                        //$days2의 길이


        if($days1==0){
                $dday = "D-day";
        }else if($dslen == 1){                          //$dslen이 한자리면 앞에 0을 붙임
                $dday = "D".$dmark."0".$days2;
        }else{
                $dday = "D".$dmark.$days2;
        }
?>

<head>
<meta name="viewport" content="user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0, width=device-width" />
</head>
<p><h2><?=$dday?></h2></p>

[설명] strtotime("2016-05-28")) 항목에 d-day 의 날짜를 입력해준다.