title: Property 'getWeek' does not exist on type 'Date' in Typescript
body: <p>I'm trying to calculate start date and end date of a week from the given date range.</p>
  
  <p>Below code gives me this error <code>[ts] Property 'getWeek' does not exist on type 'Date'.
  </code></p>
  
  <pre><code>  Date.prototype.getWeek = function(start)
  {
      start = start || 0;
      var today = new Date(this.setHours(0, 0, 0, 0));
      var day = today.getDay() - start;
      var date = today.getDate() - day;
  
      var StartDate = new Date(today.setDate(date));
      var EndDate = new Date(today.setDate(date + 6));
      return [StartDate, EndDate];
  }
  
  var Dates = new Date().getWeek();
  </code></pre>