I think this might be what you want:
case "hour" :
$hour_start = gmdate("Y-m-d H:00:00");
return array(
"start_date" => $hour_start,
"end_date" => gmdate("Y-m-d H:i:s", strtotime("{$hour_start} +1 hour")));
break;
You have to do 1 hour after the start hour. Otherwise it takes it as 1 hour from now.