computed date property on a datetime field is unreliable and polluted by the field formatter if a user timezone is set

This affects https://www.drupal.org/project/action_link → , for example - outputting a date field action link in the same place as the controlled date field is rendered (such as in a view) makes the link broken, because it gets the wrong date value from the entity field.

I debug this issue and this is the root cause for the issue:

File: DateTimeFormatterBase.php

protected function buildDateWithIsoAttribute(DrupalDateTime $date) {
  // Create the ISO date in Universal Time.
  $iso_date = $date->format("Y-m-d\TH:i:s") . 'Z';

  $this->setTimeZone($date);

  $build = [
    '#theme' => 'time',
    '#text' => $this->formatDate($date),
    '#attributes' => [
      'datetime' => $iso_date,
    ],
    '#cache' => [
      'contexts' => [
        'timezone',
      ],
    ],
  ];

  return $build;
}

here line $this->setTimeZone($date);
is the reason for saving the invalid date.

Thanks!

I can confirm that if I replace the two calls to setTimeZone() in DateTimeFormatterBase with:

    $clone = clone($date);
    $this->setTimeZone($clone);

then the problem is fixed.

I don't think that's the way to fix it though, as a child class might also call setTimeZone() and cause the same problem.

We probably need to remove setTimeZone() and replace it with a method which returns a cloned date rather than modifies the original.