PHP DateTime __construct() Failed to parse time string (xxxxxxxx) at position x

You should use setTimestamp instead, if you hardcode it:

$start_date = new DateTime();
$start_date->setTimestamp(1372622987);

in your case

$start_date = new DateTime();
$start_date->setTimestamp($dbResult->db_timestamp);

Use the createFromFormat method:

$start_date = DateTime::createFromFormat("U", $dbResult->db_timestamp);

UPDATE

I now recommend the use of Carbon