Didn't realize I needed to specify incoming timezone... using the following worked for me...
function convertTZ($date_time, $from_tz, $to_tz, $format_tz)
{
$time_object = new DateTime($date_time, new DateTimeZone($from_tz));
$time_object->setTimezone(new DateTimeZone($to_tz));
return $time_object->format($format_tz);
}