Setting #default_value for date field in a D8 form

I had a hard time getting the #default_value to work for the date form field because the examples module and the documentation uses this approach:

    $form['send_date'] = [
      '#type' => 'date',
      '#title' => $this->t("Send Date"),
      '#default_value' => [
        'month' => 2,
        'day' => 15,
        'year' => 2020,
      ],
    ];

After playing around with it for a while, I was able to get it to work by passing in a YYYY-MM-DD value, using date.formatter (the new way to format_date()).

    $date_formatter = \Drupal::service('date.formatter');
    $form['send_date'] = [
      '#type' => 'date',
      '#title' => $this->t("Send Date"),
      '#default_value' => $date_formatter->format(REQUEST_TIME, 'html_date'),
    ];

Looks like this is a known issue that has also caused others lost time. :( Hopefully this will get rolled out soon.