WPF Datepicker functionality

My DatePicker is currently bound to an xml element.  I would like the calendar date to default to the current date when going to select, and would like the text part to automatically update when another field is changed.  Not sure exactly how to go about acheiving those reactions from the DatePicker.

<DatePicker x:Name="txtBulletinPostUntil"
                      Grid.Column="1"
                      Margin="5,5,0,5"
                      Width="129"
                      HorizontalAlignment="Left"
                      Text="{Binding Bulletin.PostUntil, Mode=TwoWay}" />