controller is undefined in angular directive

Question

I need to have a custom validation for the date field in my Angular form text field. I am using the angular-ui datepicker element for the display. For the validation I have create my custom directive called validatedate in which I want to write my custom logic. There are couple of issue that I am facing:

1. The ctrl is coming as undefined inside the directive.
2. The directive should be called every time when there is a change in the input field which is currently not happening. The directive is called only once when the page is loaded.

I am adding the code snippet below for reference. Please let me know where I am going wrong.

<p class="input-group">
  <input type="text" class="form-control" name="enable_dt" uib-datepicker-popup="{{format}}"
         ng-model="enable_dt" is-open="datepickers.enable_dt" min-date="minDate" max-date="maxDate"
         datepicker-options="dateOptions" validatedate
         close-text="Close" ng-requied="maintainanceForm.enable_dt"/>
 <span class="input-group-btn">
   <button type="button" class="btn btn-default" ng-click="open($event,'enable_dt')">
     <i class="glyphicon glyphicon-calendar"></i></button>
 </span>

The validatedate directive:

'use strict';
angular.module('myApp')
  .directive('validatedate', function() {
    return {
        require: 'ngModel',
        link: function(scope, elm, attrs, ctrl) {
            console.log(scope.enable_dt);
            console.log(ctrl); //ctrl is undefined
        }
    };
});