You must be on the main phone on jack 01 to set time.  You can determine if your phone is jack 01 by pressing PROGRAM 99 on a display phone and then pushing the number 6 on the dial pad.  The phone will read the jack and extension number.  When you have determined the jack 01 phone, do the following.

Example is to change time from 3:15 PM to 4:17 PM

KEYSTROKES

(Press these keys)

Press PROGRAM * * 1234
(Default password is 1234. yours may have been changed)

Press  SP-PHONE 3 Times

Enter 2 digit hour

FWD/DND button or press the softkey below -> on the display

Enter 2-digit minute

Auto Dial/Store Button

Lift handset and hang up to leave programming mode