// failing validator rule

$validator
    ->date('date')
    ->add('date', 'unique', [
    'rule' => 'validateUnique',
    'provider' => 'table',
    'message' => "One of the dates you submitted already exists!"
]);


// working buildRules 
public function buildRules(...){

//... other rules here ...
$rules->add($rules->isUnique(['date']));

}

Error: Call to a member function format() on array
File /Users/user/Sites/clam/vendor/cakephp/cakephp/src/Database/Type/DateTimeType.php
Line: 108

https://github.com/cakephp/cakephp/issues/8001