gistfile1.php

$messageMappingConf = $this->arguments['newMessage']->getPropertyMappingConfiguration(); // there should be a getArgument as well
//$messageMappingConf->allowProperties('examdate'); shouldn't be needed
$messageMappingConf->forProperty('examdate')->setTypeConverterOption(
    'TYPO3\CMS\Extbase\Property\TypeConverter\DateTimeConverter',
    \TYPO3\CMS\Extbase\Property\TypeConverter\DateTimeConverter::CONFIGURATION_DATE_FORMAT,
    'd/m/Y'
);