I have sixteen button NEC Electra ELite Multiline... (by 4 users)

The first solution (with the link) is correct. The second (titled Solution#1) is referring to an NEC 2000 system.

The important part of the linked page is "from an attedant station". Typically the first port on the system is programmed to be a station with the "attendant' feature class. I would try it on ext 10 or 100 or 1000 or something like that first.

Here's how you adjust the time:

1. Press <TRANSFER>
2. Press <CONF>
3. Press *
4. Press <TRANSFER>
5. Press <CONF>
6. Press #
7. "CAT MODE" displayed on LCD
8. Press <REDIAL or LNR/SPD>
9. "COMMAND" displayed on LCD
10. Press 02
11. Press <RECALL>
12. Press 2
13. Press <RECALL>
14. "02>2>Current Time" displayed on LCD
15. Input correct time using military time (HH MM SS)
16. Press <CONF>