Siggi created some commands that will allow you to lock (weather or time doesn't change unless you tell it to) or unlock (regular changes in time and weather) commands. They are the following.
/ps set time l (lock)
/ps set time u (unlock)
/ps set weather l (lock)
/ps set weather l (unlock)
You can still change the time to day (/ps set time 0) or night (/ps set time 14000) or change the weather to clear (/ps set weather clear) or rain (/ps set weather rain) while
the locks are in place, it just won't change automatically.