As part of my current job, I manage IP telephony for a building with about ~600 endpoints.
In .AU we go from +10GMT to +11GMT for daylight savings which come into effect the first Sunday of October. When we swapped over the times for daylight savings we noticed that some of the phones pull time from different sources. I searched for a definitive answer but couldnt find one in time.
Here is what we experienced. We run Cisco Call Manager 4.2(3)sr3a. The 7970 and 7975’s pull time directly from the windows taskbar time. So whatever time is showing on the taskbar is the time that will be shown on the 70 and 75’s. The 7906, 7940 and 7960 are a bit different. They seemed to pull the time from the CCM Date/Time group. The DT group is influenced by the Windows time, however the DTG will apply DST automatically.
This is what we experienced. If Windows was set at +10GMT and the DTG was also set at +10GMT (both in the sydney tz) and the time was 1PM, the 7X’s would show 1PM but all other phones would show 2PM. This is because the DT group thinks hey, Windows is +10GMT but I know were in DST so i will apply another 1 hour.
We were worried about placing either the Windows or CCM time in different TZ’s for logging reasons. If something was to happen down the track, the logs still need to reflect the right time.
I believe that these inconsistencies may have been caused by a number of factors inclusive of old firmware versions and windows patches not being applied. However due to this being a production network, upgrades on the fly are risky to do.
The fix here was to apply +10GMT(Syd) to Windows, set at the right time, and also +10GMT to CCM, but place it in the Brisbane TZ. This removed the errors as Brisbane does not participate in DST, but also kept both times at +10 to satisfy the logs.
Hope this helps some one.