No default value for '#type' => 'date'?
Step 1) Install Date module.

Step 2) Enable the Date Popup module (comes with the Date module). 

Step 3) Add code like the following:

  $form['date'] = [
    '#date_label' =&gt; 'above',
    '#type' =&gt; 'date_popup',
    '#date_format' =&gt; 'm/d/Y',
    '#datepicker_options' =&gt; [
      // limits date picker so no future date can be picked.
      // remove this section if you want to allow future dates.
      'maxDate' =&gt; "+0D", 
    ],
    '#required' =&gt; TRUE,
    '#attributes' =&gt; [
      'autocomplete' =&gt; 'off',
    ],
  ];


Result: