PHP DATE_ISO8601_EXPANDED proof of concept

DATE_ISO8601_EXPANDED.php

<?php
/**
 * In this example we test formatting a date beyond 9999,
 * both with `DateTime->format(DATE_ISO8601_EXPANDED)`
 * and with `IntlDateFormatter::create`.
 * Since we cannot create a date beyond the year 9999
 * based on a textual representation,
 * we manually calculate the timestamp for our extended date.
 * Let's say we want to get the date May 11 20030,
 * we start with a base date May 11 1970 at zero hours UTC
 * and get the timestamp of this date,
 * then manually calculate the seconds to add in order to
 * get the timestamp for our extended date.
 * In this example we see that we can actually still modify
 * the date object with DateTime->modify(),
 * so instead of manually calculating the seconds
 * we could use DateTime->modify to achieve our future extended date.
 * We can even format it correctly using IntlDateFormatter::create.
 * So the only trouble is parsing a future extended date from a string,
 * but there is no trouble handling modifies or comparisons or the likes,
 * and there is no trouble with formatting the date for output.
 */


function countLeapYears($start_year, $end_year) {
    return floor($end_year / 4) - floor(($start_year - 1) / 4)
         - floor($end_year / 100) + floor(($start_year - 1) / 100)
         + floor($end_year / 400) - floor(($start_year - 1) / 400);
}

$baseTimestamp = strtotime('1970-05-11 00:00:00 UTC');

// Calculate the number of seconds from 1970 to 20030
$yearsDifference = 20030 - 1970;
$secondsPerDay = 24 * 60 * 60;
$secondsPerYear = 365 * $secondsPerDay; // Year length in seconds
$additionalSeconds = $yearsDifference * $secondsPerYear;
$leapYears = countLeapYears(1970, 20030);
$additionalSeconds += ($leapYears * $secondsPerDay); // account for leap years

// Add the additional seconds to the base timestamp
$futureTimestamp = $baseTimestamp + $additionalSeconds;

$date = new DateTime();
$date->setTimestamp($futureTimestamp);
$date->modify('+1 day');
echo $date->format(DATE_ISO8601_EXPANDED);

$formatter = IntlDateFormatter::create(
    'it_IT',
    IntlDateFormatter::FULL,
    IntlDateFormatter::NONE,
    'UTC'
);
echo PHP_EOL;
echo $formatter->format($futureTimestamp);