title: Bind DateTime to Date and Time EditFields
body: <p>i am trying to build a gui that includes editing a DateTime value of an object. The DateTime Property has  binding to a DataPicker and a normal TextBox for the Time. 
  When i change the value in the Time TextBox the value wirtten in the DateTime Property is Today with the entered Time instead of just updating the Time, preserving the original Date.</p>
  
  <p>How can i implement a Time TextBox that only changes the DateTime time but not the date?</p>
  
  <p>Current Binding:</p>
  
  <pre><code>&lt;TextBlock&gt;Time&lt;/TextBlock&gt;
  &lt;TextBox Template="{StaticResource RoundedTextBoxTemplate}"&gt;
      &lt;Binding Path="Delivery.Date" StringFormat="HH:mm"&gt;
          &lt;Binding.ValidationRules&gt;
              &lt;v:IsValidTimeRule /&gt;
          &lt;/Binding.ValidationRules&gt;
      &lt;/Binding&gt;
  &lt;/TextBox&gt;
  </code></pre>