date('d.m.Y H:i:s', (strtotime('2021-03-21 17:30:00') + (strtotime('00:30:00') - strtotime('TODAY'))));

$dateTime = '2021-03-21 21:00:00';
$add = "00:30:00";

$addParts = explode(":",$add);
$hour = $addParts[0];
$minute = $addParts[1];
$second = $addParts[2];

$sonuc =  date( "Y-m-d H:i:s", strtotime( "$dateTime +$hour hours +$minute minutes +$second seconds" ) );

echo $sonuc;