title: validate iso date but display locale date when using a date picker component
body: <p>I want to use the date picker component with localization support. I created this date picker example</p>
  
  <p>HTML:</p>
  
  <pre><code>&lt;div id="app"&gt;
    &lt;v-app id="inspire"&gt;
    &lt;v-menu 
      :value="showDatePicker" 
      max-width="290px"
    &gt;
      &lt;template v-slot:activator="{ on }"&gt;
        &lt;v-text-field 
          :value="formattedDate"
          readonly 
          clearable
          label="Date"
          v-on="on"
          :rules="rules"
          :required="true"
          @input="selectDate"
        &gt;&lt;/v-text-field&gt;
      &lt;/template&gt;
      &lt;v-date-picker 
        :value="date" 
        :locale="currentLocale"
        no-title
        @input="selectDate"
      /&gt;
    &lt;/v-menu&gt;
    &lt;/v-app&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>JS:</p>
  
  <pre><code>new Vue({
    el: '#app',
    data: () =&gt; ({
      showDatePicker: false,
      date: '',
      currentLocale: 'de',
      rules: [
        value =&gt; this.validateDate(value) || 'Invalid date'
      ]
    }),
    computed: {
       formattedDate: function () {
         const formattedDate = this.date
  
         // !! format the date based on this.currentLocale !!
  
         return formattedDate
       }
    },
    methods: {
      selectDate: function(newDate) {
        this.showDatePicker = false;
        this.date = newDate;
      },
      validateDate: function(date){
        // !! validate iso date here !!
  
        return true
      }
    }
  })
  </code></pre>
  
  <p><a href="https://codepen.io/anon/pen/ydZqxd?editors=1010" rel="nofollow noreferrer">https://codepen.io/anon/pen/ydZqxd?editors=1010</a></p>
  
  <p>The date picker component itself returns the date with iso format. I want to use it but I also want to display the locale date format for the user. Things get tricky because I want to validate the date with the iso format but the textfield uses the formatted date as its value. So when it comes to the validation the textfield passes in the formatted date but this is the <em>wrong</em> value. It should be the iso date.</p>
  
  <p>It would be cool to have something like display/value although it would make no sense for textfields...</p>
  
  <p>Is there a way to pass in the iso format to the validation rules and <strong>only</strong> display the formatted date?</p>