TimeCount.php

<?php
function time_count($step, $start = "00:00", $end = "24:00", $except = array()){
  $start = strtotime($start);
  $end = strtotime($end);
  $next = $start;
  $results = array();
  while($next < $end){
    $time = date("H:i", $next);
    if(!in_array($time, $except)){
      $results[] = $time;
    }
    $next = strtotime("+$step minutes", $next);
  } 
  return $results;
}
$except = time_count(20, "12:00", "13:00");
$res = time_count(20, "09:00", "17:00", $except);
var_export($res);

array (
0 => '09:00',
1 => '09:20',
2 => '09:40',
3 => '10:00',
4 => '10:20',
5 => '10:40',
6 => '11:00',
7 => '11:20',
8 => '11:40',
9 => '13:00',
10 => '13:20',
11 => '13:40',
12 => '14:00',
13 => '14:20',
14 => '14:40',
15 => '15:00',
16 => '15:20',
17 => '15:40',
18 => '16:00',
19 => '16:20',
20 => '16:40',
)