Since you're a newbie you first need to understand what a DateTime object is and what a date time picker object would return to you, it is a string not a DateTime.
You need to convert it to the DateTime object and then you can do the DateTime maths on it.
| in the first time I insert a date with control "datepicker",
for that you need to get a date time picker object, and pass a valid string to it or select from the UI provided.
| in the second time when i want to modify some data and i dont want to change the date it will change automatically to the default value which is 01/01/1990 00:00:00
To change it automatically, you can simply just create a new instance, and it will go back to the Min value of the DateTime, like this
DateTime dateTime = new DateTime();
Once done, you can get the Date part using a built-in function like
MessageBox.Show(dateTime.ToLongDateString());
It will show you with the date part of the dateTime object only.
I have written an article about the DateTime object and how you can use it in WPF,
Understanding DateTime struct in .NET Framework (using C#)[
^] feel free to read it and ask question if help needed! :-) Good luck.