@alanz that is true! but surely that is a proportional response to such an outragously wrong time, that is also quite unlikely to happen by accident. (times in the past are more likely) or times within 12 hours (especially: dualboot linux into windows, windows uses local timezone for system time, but unix (rightly!) uses UTC, and if you are in NZ that's 12 difference)

Another technique that's worth considering: if an incoming message has a future timestamp, delay it until that time. That way, local timestamps always make sense and you can't trick the system by setting your clock forward. (as in the various ways UI uses timestamps, mostly for ordering messages in the UI)