In Views, when using a filter on a datetime field, what is the time zone of the offset?
For datetime fields, it uses the timezone for the user viewing the page. If no user timezone is set, it uses the default site timezone.
See the relevant comment from Drupal\datetime\Plugin\views\filter\Date.php:

Date-only fields do not have a time zone associated with them, so the filter input needs to use UTC for reference. Otherwise, use the time zone for the current user.

/**
 * Get the proper time zone to use in computations.
 *
 * Date-only fields do not have a time zone associated with them, so the
 * filter input needs to use UTC for reference. Otherwise, use the time zone
 * for the current user.
 *
 * @return string
 *   The time zone name.
 */
protected function getTimezone() {
  return $this-&gt;dateFormat === DateTimeItemInterface::DATE_STORAGE_FORMAT
    ? DateTimeItemInterface::STORAGE_TIMEZONE
    : date_default_timezone_get();
}