Binding b = new Binding( \"Value\", person, \"BdayNullable\", true );
dtBirthdayNullable.DataBindings.Add( b );
b.Format += new ConvertEventHandler( dtBirthdayNullab
DateTimePickers can't be set to null because DateTime can't be null, but you can set them to DateTime.MinValue which is the default value for an uninitialized DateTime. And then you just check if the dtp.Value = DateTime.MinValue and if so, treat it as null.
However, if you want to really distinguish when no value has been selected, the easiest way is to set DateTimePicker.ShowCheckBox to true, and then you check dtp.Checked and if it's true, you read the value, otherwise you treat it as a null.