Unable to set #default_value for a date field
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',
    ]
];