$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;