MySQL Time differencials - php

Question

I am using php and mysql to create an automated html email report. This report shows the total number of emails received and logged into our system by a particular client. This is broken down by month in the given year. It also shows the average turn time from the point the email was said to be received, and the point that the ticket (ticket is logged into our database with information regarding email) is closed. Currently, I am using the following query and function to get the average turn time each month. I was told that they want to calculate the turn time based upon business hours, 8-8, not a full 24 hour day.
How can I calculate an average turn time between assuming that the time allotted in a day is 12 hours (8-8)?
If an email was received 21:00:00 Monday, and the ticket was closed 8:05:00 Tuesday, the turn time would equal 00:05:00 or 5 minutes.
$year = date("Y");
$month_num = date('m');
$total_time = "00:00:00";
for($m = 1; $m<=12; $m++)
{
$month = date('F', mktime(0,0,0,$m, 1, date('Y')));
$turn_time = GetTurnTime($month, $year);
$message .= '<td align="center">'.$turn_time['Time'].'</td>';
$total_time = sum_the_time($total_time, $turn_time['Time']);
$average_time = sum_the_average($total_time, $turn_time['Time'], $month_num);
}
function GetTurnTime($month, $year)
{
$turn_time = mysql_fetch_assoc(mysql_query("SELECT sec_to_time(AVG(SEC_TO_TIME(time_to_sec(TIMEDIFF(closed_on, received_on))))) AS `Time`, DATE_FORMAT(received_on, '%m/%d/%Y') as `Date` from `calltrak`.`calls` where monthname(received_on) = '{$month}' and year(received_on) = '{$year}' and ticket_status = 'CLOSED' and ticket_source = '2' and dept_id in (select dept_id from depts where bus_id = '4') and cust_name = 'SOMECOMPANY'"));
return $turn_time;
}
function sum_the_time($time1, $time2)
{
$times = array($time1, $time2);
$seconds = 0;
foreach ($times as $time)
{
list($hour,$minute,$second) = explode(':', $time);
$seconds += $hour*3600;
$seconds += $minute*60;
$seconds += $second;
}
$hours = floor($seconds/3600);
$seconds -= $hours*3600;
$minutes = floor($seconds/60);
$seconds -= $minutes*60;
return sprintf('%02d:%02d:%02d', $hours, $minutes, $seconds);
}
function sum_the_average($time1, $time2, $month)
{
$times = array($time1, $time2);
$seconds = 0;
foreach ($times as $time)
{
list($hour,$minute,$second) = explode(':', $time);
$seconds += $hour*3600;
$seconds += $minute*60;
$seconds += $second;
}
$seconds = $seconds / $month;
round($seconds);
$hours = floor($seconds/3600);
$seconds -= $hours*3600;
$minutes = floor($seconds/60);
$seconds -= $minutes*60;
return sprintf('%02d:%02d:%02d', $hours, $minutes, $seconds);
}