Cross-timezone calculations

timein.sh

function timein ()
{
	[ "${#}" -lt 3 ] && {
		echo 'timein <dest loc> [when] <src loc> [is] <src time>' 1>&2
		echo -e 'e.g.:\ntimein America/Chicago when Europe/Berlin is tomorrow 14:30' 1>&2
		echo -e 'or:\ntimein Asia/Ho_Chi_Minh America/New_York 2019/01/02 08:00' 1>&2
		return 1
	}

	dest=${1}; shift
	[ "${1}" == "when" ] && shift
	src=${1}; shift
	[ "${1}" == "is" ] && shift
	on=${*}

	TZ=${dest} date --date="TZ=\"${src}\" ${on}"
}