code: public function validateDatesArray($attribute, $value, array $parameters, Validator $validator)
      {
          foreach ($value as $date) {
              if (false === strtotime($date) || empty($date)) {
                  return false;
              }
          }
  
          return true;
      }
docs: Validate array of dates
  
  @param string $attribute
  @param mixed $value
  @param array $parameters
  @param Validator $validator
  
  @return bool