The question has nothing to do with
DateTimePicker
which is only used to pick some time. Generally, it's a bad idea to arbitrary set date/time on the system. You cannot provide reasonable accuracy, and so on. (I can be used to cheat copy protection systems, but newer protection is immune to such tricks. :-))
The solution can be found here:
How to set global DateTime in .net[
^].
Note that you have to use P/Invoke which compromises the platform compatibility of your code.
—SA