If you read following Angularjs
validations, you understand that:
Message will appear if user interacted and did not fill the date manually.
The problem is when date is filled using the datepicker the input is not recognized by Angularjs and still consider $invalid true, so the message remains there which is confusing/problem although date is already filled using datepicker!
<div class="form-group" ng-class="{ 'has-error' : AddForm.Birthdate.$invalid && !AddForm.Birthdate.$pristine }">
<input type="text" required data-provide="datepicker" class="form-control" name="Birthdate" ng-model="Birthdate" />
<span ng-show="AddForm.Birthdate.$invalid && !AddForm.Birthdate.$pristine" class="help-block" >
Birthdate is required.
</span>
</div>