title: How to show 'invalid date' validation message for AngularStrap datetimepicker
body: <p>I am able to validate my AngularStrap datetimepicker, but I cannot differentiate between a required validation failure and an invalid date failure.  The only error that ever shows on screen is the required error, whether it is required or an invalid string.  Is it possible in cases where a string has been entered that is invalid to show a different validation message?  Here is my code :</p>
  
  <pre><code>&lt;div class="control-group" ng-class="{error: form.BirthDate.$invalid}"&gt;
      &lt;label class="control-label" for="BirthDate"&gt;{{'_BirthDate_' | i18n}}&lt;/label&gt;
      &lt;div class="controls"&gt;
          &lt;input id="BirthDate" name="BirthDate" title="BirthDate" type="text" ng-model="user.BirthDate" data-date-format="dd/mm/yyyy" bs-datepicker required&gt;
          &lt;span ng-show="form.BirthDate.$dirty &amp;&amp; form.BirthDate.$error.required"&gt;{{'_BirthDateRequired_' | i18n}}&lt;/span&gt;
          &lt;!--&lt;span ng-show="form.BirthDate.$dirty &amp;&amp; form.BirthDate.$error.pattern"&gt;{{'_BirthDateInvalid_' | i18n}}&lt;/span&gt;--&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>What I want is something similar to the ng-pattern check but specific to the datetimepicker.</p>