如何在Cocoa应用程序中收到系统时间更改的通知？

I have a Cocoa application that records datestamps on events. I need to know when the system time is reset and by how much, but I can't seem to fine a notification anywhere that tells me such a thing has happened. This change could happen because NTP reset the clock or because the user reset (e.g. from System Preferences). It would be great if there was just a I have a Cocoa application that records datesta