[tz] Backward compatibility of tzfile?

Steve Summit scs at eskimo.com
Wed Sep 21 01:31:19 UTC 2016

Paul Eggert wrote:
> Deborah Goldsmith wrote:
>> Is the intent that successive versions of the tzfile format will be
>> compatible with older implementations of localtime.c, date.c, etc.?
> Yes, that sort of backward-compatibility has been a goal.

Case in point: the files now contain tables of 64-bit information
which allow 64-bit code to work correctly on dates past 2038-01-18.
But the files *also* contain redundant, parallel tables of 32-bit
information usable by 32-bit code.  (Naturally 32-bit code will have
problems in 2038, but there's no way around that.)  The format is
such that a version-1 (32-bit) reader will cleanly read the 32-bit
data and cleanly ignore the 64-bit extended data that it has no
knowledge of.

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