What\'s the easiest and most robust way of altering the .NET DateTimePicker control, to allow users to enter null values?
You don't need to modify it to do this.
The DateTimePicker in .net actually has a checkbox built-in.
Set the ShowCheckBox property to true.
Then you can use the Checked property to see if the user has entered a value.
http://msdn.microsoft.com/en-us/library/system.windows.forms.datetimepicker.showcheckbox(VS.80).aspx