title: knockoutjs, observables updating each other
body: <p>I have view model with 3 fields </p>
  
  <pre><code>dateStart = ko.observable();
  dateEnd = ko.observable();
  days = ko.observable();
  </code></pre>
  
  <p>assuming startDate is selected, whenever endDate is selected days field needs to be updated (days = endDate - startDate).</p>
  
  <p>Also when days field is updated i need to calculate endDate (endDate = startDate + days).</p>
  
  <p>how can this be done with knockoutjs ? </p>
  
  <p>Thank You!</p>
  
  <p>I've tried
  <a href="http://jsfiddle.net/NfG4C/6/" rel="nofollow">http://jsfiddle.net/NfG4C/6/</a>, but my js always throws too musch recursion exception.</p>