#!/bin/sh
#
# myday - print a date ranging from 6 AM to 6 AM in the ISO 8601 format
#
# SPDX-FileCopyrightText: 2023 Daniel Kalak
# SPDX-License-Identifier: GPL-3.0-or-later
#
# This script prints the current date in the ISO 8601 format. However, a
# day is defined to go from 06:00 to 06:00. So on 2023-07-01T05:30, the
# script still prints 2023-06-30. This is to ensure consistency if you
# tend to work past midnight.
#
# You can optionally specify an offset. "+0" or "-0" is the current day,
# "+1" is the day after the current day, and "-1" is the day before the
# current day. The sign is obligatory. Offsets with more than 3 digits
# are not accepted.
#
# The script returns 0 on success and 1 on bad usage.

[ "$#" -gt 1 ] && exit 1

case "$1" in
''|[+-][0-9]|[+-][0-9][0-9]|[+-][0-9][0-9][0-9]) ;;
*) exit 1 ;;
esac

date -I -d "6 hours ago ${1:-+0} day"