For gonk this is relatively simple: only gecko will change the system time, and only through the settings API, so the notification can be keyed off changes to settings.time.timezone and settings.time.msSinceEpoch (as proposed in bug 714357).

Hi Chris, This bug looks like related to Bug 717150, can I take it ? Regard Vincent

Sure! Most of the work will be defining the DOM interface in bug 714358. But we can go ahead and add the hal implementation here. We want a hal API that for listening to "clock" changes, kind of like battery-status updates. Setting the timezone and changing the system clock will trigger the notification; the notification will originate from the hal:: APIs you added in bug 717150. You should be able to follow the example of the battery-status updates for the listener notifications. Please let me know if you have questions.

This one seems duplicated by Bug 714358 which has already provided a time-changed observer at the hal:gonk level.