title: Ionic 2: cannot change date format after user sets it on date input
body: <p>I have this piece of html where there are two elements, a button and a date input. </p>
  
  <pre><code>&lt;ion-datetime id="datetime-12-0" pickerFormat="DD/MMM/YY" min="2017" max="2020" [(ngModel)]="date.date"&gt;&lt;/ion-datetime&gt;
      &lt;button (click)="triggerClick('date1Input')" ion-button="" item-end="" outline="" class="disable-hover item-button button button-ios button-outline button-outline-ios"&gt;
        &lt;span class="button-inner"&gt;{{date.date}}&lt;/span&gt;
        &lt;div class="button-effect"&gt;&lt;/div&gt;
      &lt;/button&gt;
  </code></pre>
  
  <p>I set the button to trigger the click on the date input and also set the model date.date to be printed on the button.</p>
  
  <p>Inside the component I declared the object with now date inside:</p>
  
  <pre><code>now = moment().format('DD/MM/YY');
  date = {'date':this.now};
  </code></pre>
  
  <p>The problem is: when the app starts, it shows the date correctly formated (DD/MM/YY) but when the user sets the date on the input, it changes its format to YYYY-MM-DD. </p>
  
  <p>I've already tried displayFormat, ngModelChange (after the model changed it´s value, tried to format it) but had no lucky.</p>
  
  <p>Is there anything else I could try?</p>
  
  <p>Thanks!</p>