Havent tried it but probably sendmessage to the dtp with the date/time value and then another to send a notification change message.

Maybe something like DTN_USERSTRINGA or DTN_DATETIMECHANGE


Ps, when using Spy+= you will notice that it shows the DlgControlID which never changes and is like a serial number for the control/window. So if you enum or loop you can check the return to see if its id is the one you are looking for.