[tz] tzdist and tz at iana

Ken Murchison murch at andrew.cmu.edu
Thu Sep 8 01:35:30 UTC 2016

On 09/07/2016 08:40 PM, Alexander Belopolsky wrote:
> On Wed, Sep 7, 2016 at 4:52 PM, Paul Eggert <eggert at cs.ucla.edu> wrote:
> >
> > On 09/07/2016 01:03 PM, Alexander Belopolsky wrote:
> >>
> >> That's clear, but adding an option to zic to generate timezone data 
> in tzdist format may not be out of reach.
> >
> >
> > vzic already does that, no?
> Not really.  As far as I can tell, vzic produces a single RFC 2445 
> vztimezone specification for each IANA zone.  All generated .ics files 
> have the same DTSTART set to 19700101T000000 and RFC 7808 TZUNTIL 
> property is not supported at all.

I have my own "forked" copy of vzic that I use for the tzdist service 
that I implemented in Cyrus which fixes a few bugs and plugs a couple of 
holes in the vzic that has been out there for a while. Its handles all 
rules currently in tzdata in a more compact version (RRULEs vs RDATEs) 
than what the stock vzic produces.   I intend to commit this back to 
either libical which has its own fork of vzic or back to the 
"original".  You can see an example of what it outputs here: 

Note that DTSTART is accurate per tzdata.  Also note that TZUNTIL will 
only appear if the tzdata itself has a termination date or the tzdist 
client asks for a cropped version of the time zone: 

> >  Do we need to reinvent that wheel?
> For some reason vzic author had to reinvent the wheel and write his 
> own zone file parser.  This part does not need to be reinvented.  
> Since zic already knows how to translate rules to POSIX TZ string 
> format, translating to iCalendar RRULE does not seem to require a lot 
> of code.  However, what I had in mind was that zic could implement the 
> RFC 7808 "expand" action 
> <https://tools.ietf.org/html/rfc7808#section-5.4> which is not 
> implemented in vzic.

"expand" is closer to what zdump does than vzic.  In fact, I implemented 
this format with the expand action in my server to compare against zdump 
-V to make sure that what my copy of vzic generates matches what is 

Kenneth Murchison
Principal Systems Software Engineer
Carnegie Mellon University

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