I dunno, maybe it is because American Eastern time is in daylight saving?
I am having the same problem. All of a sudden, my CZ clock was one hour late. What I did was to change my time zone from -3h to -4h, which is not the Brazilian time, but is working fine. Just a suggestion, you should set that clock with Zulu time, not EDT, and ignoring those saving changes, since Greenwich don't have them. The daylight saving time is not a thing that is used in many countries of the world, and it's not a scientifically correct time, it's just something politicians created to save on energy investments (=budget).
About Australian time, it is not just different "from the rest of the world" (=USA), as I said that is not scientific, so each country's government set up dates for the daylight saving period. The mean period is based on science, but the exact dates are defined by the politicians.