Calculate time duration in bash (only for OS X's `date`. Probably won't work with out UNIX/Linux distros)

calctime.sh

#!/usr/bin/env bash

# Usage: `calctime <start> <end>` i.e. `calctime 08:20 11:22`.
# Will prompt for start time and end time if not provided.
# DON'T specify AM or PM. Support is still @todo. 
# However, this will try its best to guess the proper duration i.e. `calctime 08:00 1:00` will print out `Duration: 5 hours`.

from=$1
to=$2

if [[ -z $from ]]; then
  read -p "From: " from
  read -p "To: " to
fi

IFS=: read from_hr from_min <<< "$from"
IFS=: read to_hr to_min <<< "$to"

secs_from=$(date -j -f "%D %T" "01/01/70 $from_hr:$from_min:00" +%s)
secs_to=$(date -j -f "%D %T" "01/01/70 $to_hr:$to_min:00" +%s)

diff=$(($secs_to-$secs_from))
if [[ $diff -lt 0 ]]; then
  secs_to=$(date -j -f "%D %T" "01/01/70 $(($to_hr+12)):$to_min:00" +%s)
  diff=$((secs_to-$secs_from))
fi

hours=$(echo "scale=2; ($diff) / (60 * 60)" | bc)

echo "Duration: $hours hours"