DATETIME_DATETIME_STORAGE_FORMAT, which is 'Y-m-d\TH:i:s'
$dateTime = \DateTime::createFromFormat('Y-m-d','2000-01-30');
$newDateString = $dateTime->format('Y-m-d\TH:i:s');
and
'field_date' => $newDateString,
https://www.drupal.org/node/2795353