<UserControl.Resources>
 <converter:ConvertDate x:Key="cvdate"/>
  </UserControl.Resources>

Public Class ConvertDate
    Implements IValueConverter

    Public Function Convert(ByVal value As Object, ByVal targetType As System.Type, ByVal parameter As Object, ByVal culture As System.Globalization.CultureInfo) As Object Implements System.Windows.Data.IValueConverter.Convert
        Return FormatDateTime(value, DateFormat.LongDate)
    End Function

    Public Function ConvertBack(ByVal value As Object, ByVal targetType As System.Type, ByVal parameter As Object, ByVal culture As System.Globalization.CultureInfo) As Object Implements System.Windows.Data.IValueConverter.ConvertBack

    End Function
End Class

<data:DataGridTextColumn Binding="{Binding DateCreated, Converter={StaticResource cvdate}}"