CheckEmptyDate is called automatically when DateTime is assigned a value that represents a time and not a date value (or Date is assigned a value of 0). As implemented in TCommonCalendar, CheckEmptyDate does nothing. 

TDateTimePicker overrides CheckEmptyDate to clear the check box associated with the calendar, or raise an EDateTimeError if the ShowCheckbox property is false. This allows TDateTimePicker to represent time values that do not include a date portion. (When a TDateTime value represents a time and not a date, the date portion is 0). 

Checked 

ShowCheckbox 

Date 

DateTime