title: in jquery ui datepicker, on the OnSelect() event is there anyway to get the previous selected date
body: <p>i have code on the <strong>onSelect</strong> event of the <a href="http://jqueryui.com/demos/datepicker/" rel="noreferrer">jquery ui datepicker</a> and i now want to only run my function if the date has changed values (so if a user selects a date that was already there in the textbox, i don't want to run this code as it will be a redundant calculation).  Here is my existing code.</p>
  
  <pre><code>$('#Milestone').datepicker({
      dateFormat: 'dd M yy',
      onSelect: calcUpdate
  });
  </code></pre>