You can apply formatting for viewing as OriginalGriff said.
Like;
this.dgv_PreviewGrid.DefaultCellStyle.Format = "D4";
Standard Numeric Format Strings
http://msdn.microsoft.com/en-us/library/dwhawy9k.aspx[
^]
<tk:DataGrid>
<tk:DataGrid.Columns>
<tk:DataGridTextColumn Binding="{Binding StartDate, StringFormat=\{0:dd.MM.yy HH:mm:ss\}}" />
</tk:DataGrid.Columns>
</tk:DataGrid>