No, there's no security restriction on changing the time. In fact, every
device has to have its time set any time the battery goes dead, so
restricting access to that capability would make commercial devices like
Pocket PCs useless.
You can change your thought process and, instead of trying to allow the user
to access the whole system, but restrict him from doing some things that you
don't like, you could take over the entire screen, disable the task bar,
etc. and, if he might, in some cases, need to set the time, provide that
capability in your application. Taking over the whole screen like this is
usually called "kiosk mode" and is covered in detail in other messages in
the archives of the group, searchable from GoogleGroups. I also pointed
someone else to a very specific thread with code earlier this morning.
Paul T.