thanks to repluy pradeep
I tried this way
(c the second & third line)
it is working fine
but i wan to know that whay it is not workin with earlier code

$date = new DateTime($jdate);

$temp =date("d", strtotime($jdate));

$startDay=(int)$temp;
//echo $jdate."*****".$startDay ;

//$date->modify("+1 day");
//echo $date->format("Y-m-d");
if($startDay<15) $Balance=1.5;
else if($startDay>14 && $startDay< 25) $Balance=1;
else if($startDay>24 && $startDay<32) $Balance=0.5;