private void dateTimePicker1_Enter(object sender, EventArgs e)
        {
            DateTimePickerFormat fmt = dateTimePicker1.Format;

            dateTimePicker1.Format = DateTimePickerFormat.Long;
            dateTimePicker1.Format = DateTimePickerFormat.Custom;
            dateTimePicker1.Format = fmt;
        }

Code Snippet - VB.NET

    Private Sub DateTimePicker1_Enter(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles DateTimePicker1.Enter
        Dim fmt As DateTimePickerFormat = DateTimePicker1.Format

        DateTimePicker1.Format = DateTimePickerFormat.Long
        DateTimePicker1.Format = DateTimePickerFormat.Custom
        DateTimePicker1.Format = fmt
    End Sub