Handle date inconsistencies between D6 and D8

The previous patch failed because it was created in a split-core setup and I forgot to pass --src-prefix=a/core/ --dst-prefix=b/core/ to git.

Attaching another patch, the only change is the diff prefixes, no code changes.

Testing fails because one of the assert is forbidding America/Indianapolis which shows up in the Backward Compatible timezones.

If the general idea of the patch seems fine I will spend time fixing the tests.

Eventually I decided to use a proper timezone instead of the backward compatible one and I fixed the dates from Drupal 6 with a migrate process plugin.

I am closing this issue as outdated. If anyone needs the old DST-less timezone, feel free to reopen it.

JFTR this is the process plugin I used, in case someone else needed it.

<?php

namespace Drupal\ao2_it_custom_migration\Plugin\migrate\process;

use Drupal\migrate\MigrateException;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\Row;

/**
 * Provides a 'FixDate' migrate process plugin.
 *
 * @MigrateProcessPlugin(
 *  id = "ao2_it_fix_date"
 * )
 */
class Ao2ItFixDate extends ProcessPluginBase {

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    if (filter_var($value, FILTER_VALIDATE_INT) === FALSE) {
      throw new MigrateException('The input value must be an integer.');
    }

    $time = (int) $value;

    /*
     * Drupal 6 interprets dates as a time plus an offset relative to GMT,
     * without considering Daylight Saving Time, while timezones used by
     * Drupal 8 do consider DST.
     *
     * This difference could result in dates interpreted differently by D6 and
     * D8 depending on the time of the year. The same time could be
     * interpreted as being in different days. This could be a problem if the
     * day is used in URL aliases or in file paths, breaking links when
     * migrating from D6 to D8.
     *
     * As a workaround, adjust of an offset all the problematic times.
     */
    $default_timezone = date_default_timezone_get();

    $d6_timezone = 'Etc/GMT-2';
    $d8_timezone = 'Europe/Rome';

    date_default_timezone_set($d6_timezone);
    $d6_date = date('Y-m-d H:i:s', $time);

    date_default_timezone_set($d8_timezone);
    $d8_date = date('Y-m-d H:i:s', $time);

    /*
     * It doesn't really matter in which timezone these are calculated as long
     * as they are in the same one, but using UTC looks a little more
     * symmetric.
     */
    date_default_timezone_set('UTC');
    $d6_time = strtotime($d6_date);
    $d8_time = strtotime($d8_date);

    // The offset will be 0 if the date is the same.
    $offset = $d6_time - $d8_time;
    $time += $offset;

    date_default_timezone_set($default_timezone);
    return (string) $time;
  }

}

Use a more generic title and reassign to the migration system, as this is now more useful as a reference for migrations.