Make sure no icons are lit, then press the B button to get to the time screen. Press A and C at the same time until you see the word "SET" on your screen. Press A to adjust (it'll adjust whatever is currently blinking) and B to advance (you'll be asked to set the date and time). Once you press B all the way through the date and time, everything will be set and the clock will start up again.
Yes, you can change the time again after it has been set.