[lug] Zoneinfo round-tripping?

[Lee Woodworth]

On 03/28/2016 09:18 PM, Chris Riddoch wrote:
> I noticed those files were there... not sure what to make of them.
> Should strftime be giving me those strings instead somehow? I spent a
> little while skimming through things in the recent tzcode and tzdata
> release to learn a bit about the "right" way to use these APIs. This
> rabbit hole looks like it goes down pretty far...

Not that I know much about timekeeping, but on my systems the
sys-libs/timezone-data package populates the /usr/share/zoneinfo dir.
Then /etc/localtime is a symlink to or a copy of /usr/share/zoneinfo/MST7MDT.
The /etc/timezone file has the relative file path of a file in zoneinfo,
e.g. MST7MDT. On my raspbian and gentoo systems, date and ls do not display
correct times until these files are set up. A starting point related to
these files might be tzset (3).

> 
> There's some oddly fascinating historical trivia, but I also feel like
> there's some diminishing returns on my time spent trying to figure
> this all out myself.  I have the feeling we've got some folks around
> here to know a lot more about timekeeping than I do.
> 
> 
> On Mon, Mar 28, 2016 at 8:17 PM, Orion Poplawski <orion at cora.nwra.com> wrote:
>> On 03/28/2016 08:09 PM, Chris Riddoch wrote:
>>>
>>> Hi, folks.
>>>
>>> This one's a bit of a puzzler for me - I'm getting similar behavior on
>>> both Debian and Suse.  It seems the zoneinfo databases don't have
>>> files (or symlinks or any sort of references) to make it possible to
>>> look up zoneinfo from all the timezone abbreviations - but it does for
>>> some of them.
>>>
>>> For example, 'zdump MST' and 'zdump EST' both return what I expected,
>>> which caused me to think something that's apparently not true, because
>>> 'zdump MDT' and 'zdump EDT' indicate a lookup failure.
>>>
>>> Should I not have expected the output of strftime's '%Z' to be a
>>> string that I could look up in the zoneinfo database?
>>>
>>> I guess I need to be using the longer strings for locations that
>>> zoneinfo *can* look up, like "America/Denver", which Suse puts in
>>> /etc/sysconfig/clock and Debian puts in /etc/timezone, and apparently
>>> doesn't need to be stored in a TZ environment variable somehow. is
>>> there a common API that can be used to look up that longer string
>>> across (at least) Linux distros?  Looking it up with strftime '%Z'
>>> suddenly looks a lot less useful.
>>>
>>
>> ls /usr/share/zoneinfo on my system suggests names like: MST7MDT, EST5EDT
>> for the daylight savings time observing zones.
>>
>> --
>> Orion Poplawski
>> Technical Manager                     303-415-9701 x222
>> NWRA/CoRA Division                    FAX: 303-415-9702
>> 3380 Mitchell Lane                  orion at cora.nwra.com
>> Boulder, CO 80301              http://www.cora.nwra.com
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> 
> 
>