Mitel SX-50

You should be at the Attendant Console to change the time.
To set date using a twelve hour clock

Press the FUNCTION key.
Dial 14.
-----"HHMM" flashes in display, asking you to enter the time.
Dial the time using a 24 hour format.
-----Example: 1:00 PM = 1300
Idle display sets should now show the time as set.