datediff.sh

#!/bin/sh
set -ef

# About --date=STRING: https://www.gnu.org/software/coreutils/manual/html_node/General-date-syntax.html

usage() {
	echo 'usage: datediff [-d|-h|-m|-s] [DATE1] DATE2' >&2
	exit 1
}

getdiff() {
	if [ $# -eq 1 ]; then
		d1="$(date +%s)"
		d2=$(date -d "$1" +%s)
	elif [ $# -eq 2 ]; then
		d1=$(date -d "$1" +%s)
		d2=$(date -d "$2" +%s)
	else
		usage
	fi
	diff=$((d2 - d1))
}

case $1 in
-d) shift; getdiff "$@"; echo $((diff / 86400)) ;;
-h) shift; getdiff "$@"; echo $((diff / 3600)) ;;
-m) shift; getdiff "$@"; echo $((diff / 60)) ;;
-s) shift; getdiff "$@"; echo $diff ;;
-*) usage ;;
*) getdiff "$@"
	day=$((diff / 86400))
	hour=$(((diff % 86400) / 3600))
	min=$(((diff % 3600) / 60))
	sec=$((diff % 60))
	echo "$diff s = $day d + $hour h + $min min + $sec s"
esac