The following code will only write 00/00/0000 00:00:00 back to the datetime field in the database. calling date works in callbackEditForm so why not callbackBeforeUpdate???
$crud->callbackBeforeUpdate(function ($stateParameters) {
$test = date('d/m/Y H:i:s');
$stateParameters->data['record_cmr_checked_date'] = $test;
return $stateParameters;
});