Comdial DSU 

Note:  Change the clock from the attendant's station -- usually extension 10, 100, or the first extension in the extension number range (i.e., 200 for 200 to 299 range , etc.) -- with handset resting on the cradle and the phone not in use.

Press INTERCOM
Dial  * # 0 1
Dial two digits (00 - 99) for year
Dial two digits (01 - 12) for month
Dial two digits (01 - 31) for day
Dial two digits (00 - 23) for hour
Dial two digits (00 - 59) for minutes
Press SPEAKER to end