Shell Programming and Scripting

shopt -s extglob # Bash Only

diff_times()
{
   local time1=$1
   local time2=$2
   typeset -i t1 t2 t

   if [[ "$time1" != ?([0-9])[0-9]:?([0-9])[0-9] ]]
   then
      echo "Invalid time '${time1}'" 1>&2
      return 1
   fi

   if [[ "$time2" != ?([0-9])[0-9]:?([0-9])[0-9] ]]
   then
      echo "Invalid time '${time2}'" 1>&2
      return 1
   fi

   (( t1 = ${time1%:*} * 60 + ${time1#*:} ))
   (( t2 = ${time2%:*} * 60 + ${time2#*:} ))
   (( t = t2 - t1 ))
   if (( t <0 ))
   then
      (( t = -t ))
   fi

   echo $t
}

start="19:22"
end="19:43"
echo "Difference between $start and $end is $(diff_times "$start" "$end")"

Difference between 19:22 and 19:43 is 21

# date +%s -d "19:22"
1194002520
# date +%s -d "19:43"
1194003780
# echo "1194003780 - 1194002520" | bc
1260
# echo "(1194003780 - 1194002520)/60" | bc
21