Applications hangs when using custom format containing one part ("ddd" or "dddd"). 

In this scenario, the FreeFormDateTime mask type is used and the ShowCheckBox property is set to true. When the user clears the date and start type and in the process of entering the date, press Left or Right key arrows, the CheckBox inside is focused which triggers the validation of the control.

In this particular case, we are using the SetToNullValue() method. To reproduce this behavior we can type a valid date in the editor area. Press the tab key to move the focus out of the control. Call the SetToNullValue() method which will set the Value property to null. Focus again on the control and type the same date again. When we tab out of the control, the date is valid and will be shown in the editor area. However, the Value property remains empty.

Here is how it looks at 200 % DPI:

The selected date is 2024 Feb 29. A custom date format is applied to the control:

When trying to change the date, an ArgumentOutOfRangeException exception is thrown.