PHP setTimezone似乎只适用于NULL值

I seriously can't figure out why it seems that a timezone conversion doesn't work if an actual time value is stored in the database. If the value is NULL this code does seem to convert the time correctly.

$tz = "America/Chicago";        
date_default_timezone_set('UTC');
$timezone = new DateTimeZone($tz);

$tmp_dt = new DateTime($row_ShowsRS['actual_datetime']);
$tmp_dt->setTimezone($timezone);
$actual_datetime = $tmp_dt->format('Y-m-d H:i:s');

The datetimes are all stored in the DB in UTC. I'm simply trying to pull these times back out of the DB and display them in the user's local timezone.