返回1970-01-01日期与会话日期PHP

Hi Can everyone help me pls? The date return 1970-01-01. Below are the codes, not sure where is goes wrong

$ssdate = str_replace("/","-",$_REQUEST['{$_SESSION['sdate']}']);
          $ssdate = date('Y-m-d', strtotime($ssdate));
          echo $ssdate;

have to changed to

$ssdate = str_replace("/","-",$_SESSION['sdate']);
           $ssdate = date('Y-m-d', strtotime($ssdate));
         echo $ssdate;

still same issue.