Example on how to set the date format of a TYPO3 DatetimeConverter

Controller.php

  public function initializeCreateAction()
  {
    $this->arguments->getArgument('mymodel')
        ->getPropertyMappingConfiguration()
        ->forProperty('mydate')
        ->setTypeConverterOption(
            'TYPO3\\CMS\\Extbase\\Property\\TypeConverter\\DateTimeConverter',
            DateTimeConverter::CONFIGURATION_DATE_FORMAT,
            'd.m.Y'
        );
  }