title: prettus validator not validate date properly
body: <p>Here is my validation rule:</p>
  <pre><code>ValidatorInterface::RULE_UPDATE =&gt; [
      'date_of_birth' =&gt; 'date_format:Y-m-d'
  ],
  </code></pre>
  <p>But if pass date like this: Year-11-24</p>
  <p>is not validating and generate error. I am  using <a href="https://packagist.org/packages/prettus/laravel-validation" rel="nofollow noreferrer">https://packagist.org/packages/prettus/laravel-validation</a></p>
  <p><strong>Error</strong> is:</p>
  <p>Could not parse 'Year-11-24': DateTime::__construct(): Failed to parse time string (Year-11-24) at position 0 (Y): The timezone could not be found in the database</p>
  <p>rather than that it must generate some error message</p>