title: What is the correct behaviour for date.setHours(-1)?
body: <p>I have the following code:</p>
  
  <pre><code>var date = new Date(2010,09,09); //09.10.2010 00:00
  date.setHours(-1); //sets the date to 08.10.2010 23:00</code></pre>
  
  <p>Calling <code>date.setHours(-1);</code> sets the date to 23:00 at the previous day (at least for Opera and Chrome).</p>
  
  <p>Is this legal?</p>