From an idle NEC telephone

Press the following keys in order

2286633#

Press HOLD               (CAT mode should show on the display)

Press the (ST) softkey

After pressing (ST) you will enter 02 on the keys followed by pressing (DE)

Next Enter 2 on the keypad, then Press (DE) again

Now enter the current time in 24 hour time format.  HH MM SS

i.e. 4:01 pm would be entered as 160100

Then press the (EXE) button.

Once you are done press the Exit soft key or lift and replace the handset of the telephone.  The time will update genbreally with in a minute.