title: how do you add a property to an existing type in typescript?
body: <p>
  I'm trying to add a property to the javascript 'Date' prototype.</p>
  
  <p>in javascript, i'd just do this:</p>
  
  <pre><code>Object.defineProperty(Date.prototype, "fullYearUTC", {
      get: function () { return this.getUTCFullYear(); },
      enumerable: true,
      configurable: true
  });
  </code></pre>
  
  <p>I thought I'd just be able to do the following in typescript:</p>
  
  <pre><code>class Date
  {
      get fullYearUTC(): number { return this.getUTCFullYear() }
  }
  </code></pre>
  
  <p>but I get the error</p>
  
  <pre><code>Cannot redeclare block-scoped variable 'Date'.
  </code></pre>
  
  <p>why doesn't this work?</p>
  
  <p><em>(please no comments on whether or not you think doing this is a good idea. this question is not about that.)</em></p>