title: ionic 3 ion-datetime could select invalid date
body: <p>I'm currently working on my project in ionic 3.</p>
  
  <p>Whether I'm using the ion date-time function, 
  the ion date-time could select the invalid date.</p>
  
  <p>Here's the step : 
  Select any value with 31 as a date, eg. select October 31 2017</p>
  
  <p>Click done button of ion-datetime picker</p>
  
  <p>Tap again on ion-datetime</p>
  
  <p>Change month from October to November without touching date or year values</p>
  
  <p>Observe that November 31 2017 gets selected which is an invalid date</p>
  
  <p>here's my ion-datetime html code : </p>
  
  <pre><code>&lt;ion-datetime displayFormat="DD MMM YYYY" [(ngModel)]="meetingDate" min="{{minMeetingDate}}" 
  max="{{maxMeetingDate}}" (ngModelChange)="onChangeStartDate()" class="font-card-content"&gt;
  &lt;/ion-datetime&gt;
  </code></pre>
  
  <p>and here's the ngModelChange method : </p>
  
  <pre><code>onChangeStartDate() {
      console.log(this.showTimeList);
      if (this.showTimeList) {
        this.showTimeList = false;
        this.showSubmitButton = false;
      }
  }
  </code></pre>