Drupal - How do I set the default value of a datetime form element?

use Drupal\Core\Datetime\DrupalDateTime;

and in form

$form['start_date'] = array(
  '#type' => 'datetime',
  '#title' => t('Start Date'),
  '#default_value' => DrupalDateTime::createFromTimestamp(time()),
);

The #default_value datetime field element expects a DrupalDateTime object, not a date string.

use Drupal\Core\Datetime\DrupalDateTime;

$form['start_date'] = [
       '#type' => 'datetime',
       '#title' => t('Start Date'),
       '#description' => date($date_format, time()),
       '#default_value' => new DrupalDateTime('1978-11-01 10:30:00', 'Europe/Berlin'),
     ];

For more information see Datetime::processDatetime() & Datetime in the API docs.

Also using Drupal 8. This discussion raises an important point - ensure that your date format matches.

I wanted to display the date as d/m/Y but that is not the default format used by the date field:

$today = date("Y-m-d");
$form['general']['endDate'] = ['#type' => 'date','#date_format' => 'd/m/Y','#title' => t('End date'),'#default_value' => $today,];

The clue was in the generated HTML:

data-drupal-date-format="Y-m-d"

The discussion states that the default could be changed but I did not validate that proposition.