[Freeipa-devel] timestamps

[John Dennis]

Dmitri and I were discussing the storage of timestamps yesterday. My 
understanding of best practice (after much investigation) is to store 
timestamps in UTC paired with the current zoneinfo (important 
clarification, *not* the local offset, rather the *zoneinfo* [1]). This 
is the only unambiguous way be able to convert to/from local time in the 
past, now, and in the future. It also has the nice property of 
preserving the timestamp in a normalized form (UTC) which can be 
compared to other timestamps and/or presented in another zone (in the 
past, now, or in the future).

However one problem I encountered as I tried to implement this was I 
could not find a standard library which returned the current zoneinfo. 
glibc and other posix libraries provide offsets and/or the TZ value 
(which really just an offset in wolf's clothing). I have a very clear 
understanding of how to retrieve the zoneinfo in a Linux environment. In 
fact I found a number of examples of code which had their own private 
function to query the zoneinfo. The downside to this is it is OS 
specific (but it's not too terrible to tweak it to a particular OS).

So I have a few questions:

* Jakob, in the policy engine when you wrote the code which deals with 
time evaluation how did you handle the zone issue?

* Have others needed to deal with timestamps and if so what has been 
your methodology?

* Is anyone aware of a library which can retrieve the zoneinfo in a 
portable manner?

[1] If you're wondering why offsets are evil and why zoneinfo was 
invented it's because timezones are notoriously fluid, they are the 
result of political decisions and vary in unpredictable ways (in both 
the geographic domain and the time domain). The only unambiguous way to 
know what a proper offset is to know a geographic location at a specific 
moment in history, hence the introduction of the zoneinfo database which 
allows one to query the offset based on location and moment in time.

-- 
John Dennis <jdennis at redhat.com>

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