title: Saving and retrieving Date from local storage
body: <p>This may seem like a silly question but I'm having a rather difficult time, understanding Typescript. 
  I have following code:</p>
  
  <pre><code>var date = new Date();
  window.localStorage.setItem("date", date);
  </code></pre>
  
  <p>As you see, I'm generating todays date and store it via local storage.
  Now I want to retrieve this value inside another function, add 7 days to it, store it again, and show the new value in an alert-box.</p>
  
  <pre><code>var date = window.localStorage.getItem("date");
  date.setDate(date.getDate() + 7);
  window.localStorage.setItem("date", date);
  alert(date);
  </code></pre>
  
  <p>When I run this code, it keeps telling me <code>Undefined is not a function</code> on the second rule of the second code-block (probably the .getDate() function).</p>
  
  <p>Someone who knows what I might be doing wrong? I thought this simple piece of javascript would run fine in typescript without changing the code...</p>