Combing a date column and separate time column into one result

How do I take separate date and time columns into one field that is then sent to the entity for save

$date = $data['ORDERDATE'] + $data['ORDERTIME'];
$data['ORDERDATE'] = date('Y-m-d H:i:s', strtotime($date));

The code you posted seems like it would just double up on the valued (since it literally slams them together).
As far as I know, you can’t really “combine” them as-is.
What are the input values and what do you expect to come out?