To reproduce: if (e.KeyData == Keys.Back) { radDateTimePicker1.SetToNullValue(); e.Handled = true; } 

Workaround: void radDateTimePicker1_KeyUp(object sender, KeyEventArgs e) { if (e.KeyData == Keys.Back) { radDateTimePicker1.SetToNullValue(); } }

Resolution: 
Users should handle also KeyPress event (and KeyDown event) because the keys.Back is processed on KeyPress  :

         void radDateTimePicker1_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (e.KeyChar == '\b')
            {
                this.radDateTimePicker1.SetToNullValue();
                e.Handled = true;
            }
        }