I believe that the Partner ACS uses AC line frequency to drive the clock.
There is speculation that the "J" patch for the R7 will allow the time to be adjusted by caller ID data sent from the LEC.
Realizing that consumer telephones adjust their displays (but in most cases not their answering machine clocks) from caller ID records, it is curious as to why it took this long for AT&T, Lucent & now Avaya to provide this feature on the Partner ACS. Seems like in that regard, a $50.00 VTech/AT&T telephone is more advanced than a $700.00 Partner business telephone system.
Avaya has said with the proliferation of cell phones, computers, pagers, radios & wall clocks in the workplace, the need for a dead accurate clock (Stratum 1, 2, or 3) on the telephone isn't a high priority.