Yeah. I was convinced of that 
We go step by step.
AllowNumberNullValue = True
The edit field is empty.
1st image
I enter day = 6 Value '6' IsValid = True Date = 12/30/1999
2nd image. The test for IsValid has been run
day = 6 Value '6' IsValid = True Date = 5. 1. 1999 ????
3rd image
I enter month = 4
day = 5 month = '' Value '5.. 1900' IsValid = True Date = 12/30/1999
4th picture
Test for IsValid
day = 5 month = '' Value '5.. 1900' IsValid = True Date = 8/17/2023 20:31:58
The next command raises an exception. Why?
Do you consider this normal?
Just put the component on the form, connect to the DB and define the OnValueValidate function. And you can try it.
I don't know of any description of it anywhere. If there is, where can I find it?