Format date to be saved in Drupal.

drupal_format_date.php

<?php

  /**
   * Format a date to be saved in Drupal.
   *
   * @param string $date
   *
   * @return string
   */
  private function formatDate($date) {
    $date_time = \DateTime::createFromFormat('Y-m-d', $date);
    return $date_time->format(DATETIME_DATE_STORAGE_FORMAT);
  }