Note well:
Even if you do synch-up using OpsMan and an NNTP time source, the time displayed by the phone will not always be perfectly in-synch with the rest of the world around you.
The phones have a free-running clock which appears to only re-synch with the main control when you go off hook.
The main control also has a free-running clock which itself only re-synchs with the outside world when sent a timeset signal from OpsMan (or the TIME mtce command).
Example, right now at 11:06AM the phone on my desk is already 20 seconds slow behind NBS time although the OpsMan resynch occurred at 5AM.
Most terminal emulator programs, i.e., ProCom, etc. support a script language that will allow you to automate the time-setting process and even do it as often as you like, say for example at 6:55AM and again at 11:55AM and then again at 3:55PM so at or near to "employee critical" times the SYSTEM clock will be as accurate as we can possibly make it. This leaves only the problem with the internal clock within the phone. Of course in a normal business environment we shouldn't have to concern ourselves so much with that component.
Don't depend on your phone system to be a Swiss Army Knife. If you need an accurate timepiece, buy one.