I need to know as if he puts checkbox empty.

Using The Checkbox to Select No Date
The CheckBox property makes it possible to specify whether the control returns a date. By default, CheckBox is set to False and the control always returns a date.

To enable the user to specify no date, set the CheckBox property to True (for example, if you are using the DateTimePicker to enter the completion date of a project but the project has not yet been completed).

When CheckBox is set to True, a small check box appears in the edit portion of the control to the left of the date and time. If the box is not checked, the Value property returns a null value. If the user checks this checkbox, the control returns the displayed date through its Value property.

I need to know as if he puts checkbox empty.

It stops for with null value, that is, to put check empty, is only enough to dtpiecker = null