my-element.html

<!-- src/my-element.html -->

<link rel="import" href="../bower_components/polymer/polymer-element.html">

<dom-module id="my-element">
  <template>
    <h1>Hello, World! It's [[today]].</h1>
  </template>
  <script type=”module”>
    // Heyyyy, we're pulling in a Node module!
    import format from 'date-fns/format';
    
    class MyElement extends Polymer.Element {
      static get is() { return 'my-element'; }
      static get properties() {
        return {
          today: {
            type: String,
            value: function() {
              return format(new Date(), 'MM/DD/YYYY');
            }
          }
        }
      }
    }

    window.customElements.define(MyElement.is, MyElement);
  </script>
</dom-module>