Shell Programming and Scripting

#!/ust/bin/ksh

TimeDiff()
{
   integer seconds
   integer h1 m1 s1 time1
   integer h2 m2 s2 time2

   # Convert times to seconds

   echo "$1" | IFS=':' read h1 m1 s1
   echo "$2" | IFS=':' read h2 m2 s2
   (( time1 = h1*3600  + m1*60 +s1 ))
   (( time2 = h2*3600  + m2*60 +s2 ))

   # Compute diff times in seconds

   (( seconds = time1 - time2 ))
   (( seconds < 0 )) && (( seconds = -seconds ))

   # Output result as seconds

   echo $seconds

   # Output result as hh:mm:ss

   (( h1 = seconds / 3600 ))
   (( m1 = seconds / 60 % 60 ))
   (( s1 = seconds % 60 ))
   printf "%02d:%02d:%02d\n" $h1 $m1 $s1
}

TimeDiff $1 $2

$ sh difftime.ksh 12:12:12 11:10:09  
3723
01:02:03