UNIX for Dummies Questions & Answers

Hi Guys,
I am using a command

$ runprt_req PPGUS_ROYXN1102
Output:
**************** Start Date and Time End Date and Time***
PPGUS_ROYXN1102 01/15/2008 02:20:08 01/15/2008 04:54:50
PPGUS_ROYXN1102 01/12/2008 02:03:57 01/12/2008 04:22:10
PPGUS_ROYXN1102 01/11/2008 02:07:55 01/11/2008 04:28:54
PPGUS_ROYXN1102 01/10/2008 03:13:57 01/10/2008 04:52:24

I need to find the average time between Start and End Time
Ex: 02:20:08 - 04:54:50 So the average time is 02:34:02

Similarly i need the average time for the all the columns
Ex:
Start Time End Time Average Time
02:30 02:40 00:10
03:30 03:30 00:00
...........
I have a very big output, so its difficult to find the average time easily.
It will be helpfull for me, if i get a command or a script.
I tried using awk command, but i not much familiar with unix, so i didnt get any output except Syntax Error
Thanks in Advance.

Code:

#!/bin/sh
awk 'NR>1{
        n=split($3,starttime,":")
        m=split($NF,endtime,":")
        s_hr=starttime[1]
        s_min=starttime[2]
        s_ss=starttime[3]
        e_hr=endtime[1]
        e_min=endtime[2]
        e_ss=endtime[3]
        stime = (s_hr * 60 * 60) + (s_min * 60) + s_ss
        etime = (e_hr * 60 * 60) + (e_min * 60) + e_ss
        timedifference = etime - stime
        print timedifference " secs"
        # I leave to you to calculate back the average time in hr:min:sec
        # hint: take timedifference divide by 3600 to get hr. convert the remainder to min and secs

}
' "file"