Been working on this and finally got a result although I don't know how to implement this into the schedules if that's what you want:
<?php
$min_time = 6;
$max_time = 21;
$random_hour = rand($min_time,$max_time);
$number_formatted = str_pad($random_hour, 4, "0", STR_PAD_BOTH);
if ($random_hour < 10){
echo $number_formatted;
}
else
{
echo number_format($random_hour,2,"","");
}
?>
$min_time is the earliest the flight can leave and max is the latest. Hope this helps