#!/bin/bash

yell() { echo "$0: $*" >&2; } # print script path and all args to stderr
die() { yell "$*"; exit 111; } # same as yell() but non-zero exit status
must() { "$@" || die "cannot $*"; } # runs args as command, reports args if command fails
dateDuration() {
    # Desc: Given two date strings, output seconds duration
    # Example: dateDuration 2012-06-30 2012-07-01  # Yields 86401 because leap second
    # Version 0.0.4
    # Depends: GNU Coreutils 9.4 (date, sed), GNU Bash 5.2.21
    # References: [1] “Understanding the "right" time zone database” https://kenta.blogspot.com/2016/03/sqfzcxay-understanding-right-time-zone.html

    local date1 date2 re unixEpoch1 unixEpoch2 duration;

    path_tz="/usr/share/zoneinfo/right/UTC";
    if [[ ! -f "$path_tz" ]]; then yell "WARNING:Time zone 'right/UTC' file not found at ${path_tz}. Try 'sudo apt install tzdata-legacy'."; fi;
    if [[ $# -ne 2 ]]; then die "FATAL:Need two date arguments. Got $#"; fi;
    date1="$1";
    date2="$2";
    # Check args
    if [[ -z "$date1" ]]; then die "FATAL:Invalid first date provided:${date1}"; fi;
    if [[ -z "$date2" ]]; then die "FATAL:Invalid second date provided:${date2}"; fi;
    
    ## Convert @-specified unix epoch into right unix epoch (rue)
    re='^@.*';
    if [[ "$date1" =~ $re ]] || [[ "$date2" =~ $re ]]; then
        die "FATAL:@-specified unix epoch detected. Why are you using me?";
    fi;

    # Convert date strings into Unix epoch. # See [1]
    unixEpoch1="$(TZ=right/UTC date --date="$date1" +%s)" || die "Problem with $(declare -p date1)";
    unixEpoch2="$(TZ=right/UTC date --date="$date2" +%s)" || die "Problem with $(declare -p date2)";

    # Calculate duration
    duration="$((unixEpoch2 - unixEpoch1))";
    duration=${duration#-};

    printf "%s\n" "$duration";
};

# Examples
dateDuration 2026-06-30 2026-07-01; # 86400 seconds
dateDuration 2012-06-30 2012-07-01; # 86401 seconds (leap second)
dateDuration 2000-01-01 1999-12-31; # 86400 seconds
dateDuration 2012-06-30T23:59:59+00 2012-06-30T23:59:60; # 1 (leap second)