There was a power outage the time on the phone was incorrect when I tried to set the time it went back to 12 am and changed the date to Sun. How can i reset the date and time on the Avaya 18D phone?

You may have had a power outage or surge that defaulted all your set-up info and fax settings to factory settings; or even possibly damaged the circuit board.
Here's a link to download the service repair manual for your fax machine: http://diagramas.diagramasde.com/otros/UX340L.pdf

5 8 GHz Expandable Digital Cordless Answering System with Two Handsets

Possible issue if you have Caller ID:

Why does the date and time change automatically after I have programmed the date and time on my phone?

If you are subscribed to Caller ID from your phone company, the unit's date and time setting will be automatically adjusted when caller information is received. If you do not wish for this feature to work, press [MENU] [#] [2] [2] [6] on the handset, then press [0] to select "Manual" and press [SAVE]

ftp://ftp.panasonic.com/telephone/om/kx-tg5631_en_om.pdf

Page 21 - setting date and time in manual at link above.

When you pull the crown out there are two positions. The first is the day/date setting position. Turn the crown one way and the date changes, the other direction the day of the week changes. Set both the day and date to yesterday. It will become evident why when you finish. Now pull the crown all the way out and notice when you turn the crown the hands move. Advance the hands to the 12 o'clock position in a clockwise direction. As you pass 12 the date may begin to change. If not then you now know that was noon and not midnight because the date only changes at midnight. If the date did not change advance the hands all the way around 12 hours. This will now be midnight and the date should begin to change. It may be almost instantaneous or may take until 12:30 or so. Continue advancing the hands to 4 o'clock. You should see between midnight and 4 the day of the week advance. Now the day/date are correct for today and the final step is to set the correct time. Remember, you are now set a 4 AM. Advance the hands to the correct time. That means if it is 3 in the afternoon, you must advance past noon to 3. You cannot go backwards. That would be 3 AM!

on a phone
(not cell phone ) when you set something you go to menu then exit and it is the exit that makes the saving

I have posted a video which shows step-by-step how to change the date and time, written version is available at http://www.abbeytelecom.com
I hope that this helps

you must first disable Autoupdate (turn it off) - go to settings>scroll to Initial Setup, (select this)>Time & Date (select this)>Autoupdate (set to OFF)>after you do this option will appear to change time to 12/24 hour....

Try this:
1.) Make sure the answering system is on.
2.) Press flash/answerer/call waiting button on the handset to enter answerer remote mode.
3.) Press key (8) to enter Time/Day set menu using the handset.
4.) To set the Hour, key in (00)-(12), like 08 for 8 o'clock.
5.) To set Minutes, key in (00)-(59), like 15 for 15 minutes after the hour.
6.) To set AM/PM, press the CALLER ID-VOLUME up or down buttons.
7.) To set the day of the week, press the CALLER ID-VOLUME up or down buttons.
8.) Press #PAUSE/ringer button to confirm the setting.

Hope this helps you out! Hal

Hi.
Turn key to P mode
MMDDYY then day of week code (0=sun,1=mon etc) then X/time.
E.g Sat june 12 1993 = 0612936 X/time

Mark

Go to your Bios Settings. Change the System Date there.

RESETTING THE CLOCK AFTER A TOTAL MEMORY LOSS

If you do not change the built in lithium battery within a reasonable period of time after you see the ‘flashing’ LOW BATT. warning indicator, the lithium battery may become exhausted and will no longer be able to supply back up power to the SmartSet internal computer.

If the unit is disconnected from the AC outlet after the battery is exhausted, or if a power interruption occurs, the SmartSet memory will be lost and the clock will need to be reset when the power is restored. Be sure to install a fresh lithium battery before you reset the clock, otherwise the clock will need to be reset each time the power is interrupted.

If the lithium battery is exhausted and you experience an interruption in the AC power supply, the clock will reset itself to its initial default setting when the power is restored.

The initial default setting is “12:00 AM, Saturday, January 1
(1 1), 2005”.

If you notice that the time on your SmartSet clock radio is completely incorrect, and the day indicator is also wrong, press the MONTH/DATE button to see the date. If the date display is showing “1 1” (January 1 ), you probably experienced
a total memory loss. To reset the clock after a total memory loss, proceed as follows:

1.) Install a new CR2430 lithium battery in the battery compartment.

2.) Connect the radio to your AC outlet and let the clock set itself.

3.) Depress the MONTH/DATE and TIME buttons together. The display will change from ‘time’ to ‘year’. While holding the MONTH/DATE and TIME buttons, press the UP button to
set the display to the correct year, then release the buttons.

4.) Depress the MONTH/DATE button. The display will change from ‘time’ to 'month/date’ (mm/dd). While holding the MONTH/DATE button, press the UP button to set the display
to the correct month and date. The Day indicator will change automatically as you adjust the month/date settings. When the correct month and date appear on the display, release the buttons.

5.) Depress the TIME button and the UP button to set the display to the correct time. Be sure to confirm that you have set the time correctly to ‘AM’ or ‘PM’ by observing the AM indicator.

6.) After you set the time, reconfirm that the Day, and Month/Date displays are still correct. Make final adjustments if necessary.

The SmartSet clock memory is now reset and the new lithium battery will maintain it for the next 5 to 8 years, depending on the number and duration of the power outages you experience.

IMPORTANT NOTE:
After resetting the clock, don’t forget to reset your wake up times, and alarm mode settings, and sleep timer settings as well.