#!/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 0.0.1
    # Depends: GNU Coreutils 9.4 (date, sed), GNU Bash 5.2.21
    # References: [1] “"right" tz database (zoneinfo) files and GPS-based NTP” https://www.ucolick.org/~sla/leapsecs/right+gps.html

    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;
    if [[ $# -gt 2 ]]; then die "FATAL:Too many arguments."; fi;
    ## Convert @-specified unix epoch into right unix epoch (rue)
    re='@.*';
    if [[ "$date1" =~ $re ]] || [[ "$date2" =~ $re ]]; then
        die "FATAL:@-specified unix epoch detected. Why are you using me?";
    fi;

    # Use right unix epoch to account for leap seconds.
    export TZ=right/UTC; # See [1]

    # Convert date strings into Unix epoch
    unixEpoch1="$(must date --date="$date1" +%s)";
    unixEpoch2="$(must date --date="$date2" +%s)";

    # Calculate duration
    duration="$((unixEpoch2 - unixEpoch1))";
    duration="$(sed -e 's/^-//' <<<"$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)