Unable to set #default_value for a date field I have a form with a date field.

$form['filter_dates']['start_date'] = [
    '#type' =&gt; 'date',
    '#title' =&gt; $this-&gt;t('From'),
    '#description' =&gt; $this-&gt;t('Start event date'),
    '#default_value' =&gt; [
        'year' =&gt; 2015, 'month' =&gt; 05, 'day' =&gt; '09',
    ]
];


But when the form is displayed the date field doesn't contain the value "09.05.2015". The date field is empty.

Could you clarify me, how can I fill the date field with the value ?
I watched it as on my form as on the examle form from the Examples module.
Set it like this

$form['filter_dates']['start_date'] = [
    '#type' =&gt; 'date',
    '#title' =&gt; $this-&gt;t('From'),
    '#description' =&gt; $this-&gt;t('Start event date'),
       '#default_value' =&gt; [
        '2015-05-09',
    ]
];