the value in calendar change to default value

The Keys method doesn't mind different characters. It just inserts whatever you give as if it was being typed in.

https://support.smartbear.com/testcomplete/docs/reference/test-objects/members/common-for-sys-desktop-pda/keys-method-sys-desktop-object.html

i changed the format of date and time and then instead of store in a variable i used the direct in test 

date_fin_constant = aqObject.GetPropertyValue(Aliases.....DateEdit.TextBoxMaskBox, "wText");

and i used the keys methode and worked 🙂