Help With Understanding the Binary Files

Olson, Arthur David (NIH/NCI) [E] olsona at
Mon Dec 14 19:09:44 UTC 2009

For nearly all purposes, the UTC/local and standard/wall indicators can indeed be ignored. They're only important when a file is used (by creating a link from "posixrules" to it) as the basis for handling POSIX-style TZ environment variables; they control how the instants stored in the file are mapped to the instants when DST begins and ends.


-----Original Message-----
From: Bill Seymour [mailto:stdbill.h at] 
Sent: Saturday, December 12, 2009 10:57
To: tz at
Subject: Re: Help With Understanding the Binary Files

I've finally gotten around to doing some analysis on actual
zoneinfo information files; and I've determined empirically
that tzh_is***cnt are either zero or tzh_typecnt as Guy Harris
recently said.

But I've also found lots of files, not just poxisrules, in which
the UTC/local and/or standard/wall indicators exist and are
non-zero; so I'm still confused.  Do I simply ignore them
in all files except poxisrules (as Harris seemed to imply,
but I might have misread it), or do I need to reinterpret
the transition times based on these indicators when they

(If the latter, that seems to be the less desirable design.
Since the files are used much more often than they're
created, wouldn't it be reasonable to compute the wall
clock times eagerly rather than lazily?  Note also that,
while creating the files, we have enough information
to do that correctly.  At the time the file is read, we've
lost the DST offset amount and have only a switch
indicating /whether/ we're observing DST.)


--Bill Seymour

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