In general, time zone rules are ``until further notice'', and any automated system for propagating time zone rules will just have to take this into account.
Well, with for instance the EU rules, we have in the law defined for how long the current rules are guaranteed to last. This knowledge is useful for a time zone handling system if redundant information about the time zone is available, in order to adjust the priorities of the redundant information according to the expire date and in order to decide when to bother the user with warning messages. For instance, in the Newman draft for future dates, we have both a tz name like Europe/Berlin and a numeric offset like +02. If the receiving system has a rule for Europe/Berlin, then it should apply it if the rule has not yet expired. If the rule has expired and what the rule says contradicts the given numeric offset, then the user should be informed in a warning about the problem that there might be a missunderstanding regarding the time zones on the two communicating systems. This leads me to a new suggestion: The numeric offset in the Newman draft should be extended by a "?" flag that indicates whether the given future time is AFTER the expire time of the time zone rule as known by the sending system. For instance 2010-09-30 15:00 Europe/Paris +02? means that the sending system assumes that France will still be in UTC+02h at this time, but it is not sure since the EU directive it knows goes only until 2004. If the receiving system has a later expire time (due to updated tables), it can due to the ? found after the offset silently override the numeric offset and has not to bother the user with a warning just because the sending side has old tables. (All this is said with the distributed calendar applications in mind. I am not sure however whether it is reasonable to build that sophisticated mechanisms to handle the uncertainties of future local time stamps, or whether this just leads to annoying software that causes problems by trying to be too smart.) Markus -- Dipl.-Inf. Markus Kuhn, Schlehenweg 9, D-91080 Uttenreuth, Germany mkuhn at acm.org, http://wwwcip.informatik.uni-erlangen.de/~mskuhn