Initialize the DateTimePicker control to a blank value by setting the Format property to Custom and the CustomFormat property to a space (" "):
DateTimePickerConstDate.CustomFormat = " "
DateTimePickerConstDate.Format = DateTimePickerFormat.Custom
Then, when the value changes, change the Format property back to its original value (or define a usable Custom Format:
Private Sub DateTimePickerConstDate_ValueChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DateTimePickerConstDate.ValueChanged
DateTimePickerConstDate.Format = DateTimePickerFormat.Short
End Sub
For more information on this solution and other approaches, see the following:
http://social.msdn.microsoft.com/Forums/en-US/netfxcompact/thread/46b2a370-72ec-485c-9361-bfa505bb6863/