KEP 2: Modification of datetime type, introduction of 'tz' sub-tag

Gunnar Wrobel wrobel at
Tue Dec 7 22:33:45 CET 2010

Zitat von "Georg C. F. Greve" <greve at>:

> Hi Hendrik,
> On Tuesday 30 November 2010 10.52:45 Hendrik Helwich wrote:
>> For backward compatibility i agree that clients should be able to read
>> RFC3339 for the first koab format version. But for writing i think clients
>> MUST write a clear normailzed datetime format like the Zulu format and
>> also do not need to read RFC3339.
> If clients can read it for ONE Kolab version, they can read it for all of
> them. I don't think anyone would really want to have multiple parsers for two
> formats where one format is a limited subset of the other.
>> This is because i see no real benefit for kolab in the complex RFC3339
>> datetime format. You can have partially timezone information in that
>> datetime format. And there is no need for this information. Why not omit
>> this unused information to make the format more clear and remove
>> redundancy?
> As another discussion demonstrated, it is not actually time zone information,
> just an offset. While - as long as all times are always stored without DST in
> effect - it allows to extrapolate some idea about the meridian, it does NOT
> allow to extrapolate to the correct DST regime.
> So it is a pure offset, and *not* a redundancy.
> But as an offset it is fairly harmless, and parsers can be expected  
> to have no
> issues deal with it. So it does not introduce any risk or instability.
> But you are right it is also not strictly necessary.
> If anyone has an idea why it ended up in the format, I'd be interested in the
> story. My guess is that it somehow came from ISO8601, from which RFC3339 was
> then derived as a schema, IIRC.
>> If we have the possibility to change things in version 1.1 i would like to
>> pick up the idea of Andrew (Mail from 12.11.2010) to specify times directly
>> in local time e.g. for the element "last-modification-date" in the Zulu
>> format and for the element "start-date" optionally in a local time like
>> this: [...]
>> So the Suffix 'Z' could indicate that it is a UTC time. For a local time,
>> the timezone which is specified in the kolab format xml must be used.
> Is this RFC3339 compliant?
> It seems like it would establish a third, RFC3339 incompatible format, which
> would be very easy to mistake for an RFC3339 compatible format, because it
> probably defeats the expectation of many client implementors that someome
> would go to create yet another time format when RFC3339 is widely used for
> this purpose.
> That seems like a rather risky proposition to me.
>> You suggested that the Zulu format could be not sufficient. I think you are
>> referring to the milliseconds. Do you see a use case where this could be
>> needed?
> We are planning to extend Kolab integration into the area of real time
> collaboration technologies, among other things, including collaborative
> editing and such.
> It is entirely foreseeable that such applications would be based  
> upon RFC3339,
> so if RFC3339 is not supported it would necessitate translation which would
> introduce yet another potential source for error with no gain, and we might
> find that some of these applications actually make use of the milliseconds.
> So yes, I would like to not close that door.
> As a compromise proposal - because strict Zulu UTC with the time zone
> information is sufficient for the purposes of all existing Kolab objects - we
> could say that
>  * Clients *MUST* be parsing datetime as RFC3339
>  * As a general rule, Clients *SHOULD* always write datetime in the simplest
> possible format
>  * For all existing objects individually we specify UTC Zulu *MUST* be used.
> This way we'd keep backward compatibility, and older clients will be able to
> continue reading the timestamps unless they have implemented the implicit "do
> not read versions above your level" rule.
> At the same time we ensure that we have a smooth path to integrate other
> technologies, including those for which we'd encounter the current  
> approach to
> fall short.
> Because there is nothing yet that uses it, clients would gain a grace period
> to switch to full RFC3339 parsing where they aren't already using it.
> What do you think?
>> So in fact i think we have to do a trade-off here and decide what is more
>> important:
>> (a) to use actual time zone data
>> (b) to assure that all people come to the same time to a meeting
> You are right there is a trade off here. But this is not it.
> With static time zone specifications, some people will still miss their 11:00
> meeting because they know it's at 11:00, it has been there for years. So when
> the computer tells them it is now at 10:00, they will probably ignore the
> computer, and still go at 11:00, just like they'll ignore their car navigator
> that tells them to turn right when they know they need to go straight.
> If that same guy's time zone file is out of date, but everyone  
> else's is still
> up to date, he'll just as gladly go to the meeting at 11:00, and  
> meet everyone
> as planned, and all is well.
> What I am not trying to say is that ALL people would behave like this, some
> people will behave differently. What I am trying to explain is that *NONE* of
> these options can *GUARANTEE* that everyone will be at the right time at the
> right place.
> The questions are:
>  (a) Which method has the better chance of providing correct information?
>  (b) Which method is more robust against future developments?
> and, of course,
>  (c) Who will tech support (and consequently the user) blame for the failure?
> The answers to these questions are fairly straightforward.
> Answer to (a): As long as DST rules do not change, both perform equally well.
> Once DST rules change, the static encoding *will* break, the database *may*
> break, but only if the user has not updated their system in quite a while,
> because such changes are prepared politically, then communicated,  
> incorporated
> into the database and made available quickly.
> So in most cases there is a substantial update window which would only be
> realistically missed in an unmaintained, unserviced and essentially orphaned
> installation of Kolab. I'd expect those users to have much bigger problems
> than a recurring meeting that switched one week early or late.
> Answer to (b): The database.
> Because there is an RFC in the works that will do for DST  
> information what RFC
> 1305 did for time synchronization. Just like there are very few people today
> who use atomic clock receivers to set their system time and instead rely on
> NTP, only very few people will use database updates, and instead  
> rely upon the
> network service.
> Answer to (c):
> For the static approach: Kolab.
> For the database: The platform provider.
> So the database scores better on every single issue. That is why, even though
> it is not (yet) perfect, I see it as the better of two imperfect choices.

Finally found the time to read through the Olson/static part and make  
my mind up about it. This part from Georg has been the perfect summary  
for me and Olson definitely gets a big +1 from my side.

To add the PHP/Kolab server perspective: The Olson db has been part of  
PHP since 2005 and is being updated regularly. If you don't update PHP  
respectively your Kolab server for three to five years (as has been  
discussed here) the Olson database and recurring events crossing DST  
is indeed the least of your problems ;) I would say the same holds  
true for clients.

Still I can imagine that people might actually keep a system without  
updates for years. "Never change a running system" is sensible after  

*But* ... if this is causing problems then it can be easily fixed by  
updating the problematic system. Static information on DST in the  
format would mean that you could only fix the problem by trying to fix  
problematic events in the Kolab IMAP storage. This is *way* harder and  
nothing you want to go into.



>> Additionally it could be allowed for clients to update the timezone data in
>> the kolab item if they notice that its outdated.
> That only seems to combine the weaknesses of both approaches, and none of the
> strengths, while increasing the burden on client implementors and complexity.
> It adds many questions to which we'll have to discuss answers, such as:
> When does one client determine that the static information is out of touch
> with reality? How does it ensure its update is the better one, and won't be
> overwritten by another client which may or may not correctly see  
> that the first
> client was wrong in its assumption? How do we ensure that all clients have
> received the latest updates before displaying the event to their user?
>> > So it will likely be supported by an NTP-like service to update DST
>> > information in the future, which will give us maximum reliability and
>> > assurance of correct display, with no change to the storage format.
>> To be honest i have doubts that this will work correctly in practice.
> This is a much simpler problem than NTP.
> Considering that NTP seems to work pretty well in practice, I have little
> doubt this can be made to work, to be honest.
>> All people on all the different systems need to have always a database which
>> is fully mappable to Olson database and which also needs to be in the same
>> state. How can it be assured that all people always update their database
>> at the same time?
> Firstly, they don't need the SAME database. They only need a version of the
> database that correctly gives DST rules for the current instance.
> If rules have not changed in the region in question, but only in  
> another which
> is irrelevant to the calculation at hand, different versions will deliver the
> same result.
> Secondly, when limiting it to "versions of the database that are correct with
> regards all the zones relevant to the calculation", not everyone will need to
> have them, only those providing that service.
> A cron job of importing the latest version once a day ought to do  
> just fine for
> that. Remember: The Olson database changes very infrequently, and for events
> that are months in the future. So as long as there is at least one update per
> half-year, you're typically on the safe side.
> Best regards,
> Georg
> --
> Georg C. F. Greve
> Chief Executive Officer
> Kolab Systems AG
> Zürich, Switzerland
> e: greve at
> t: +41 78 904 43 33
> w:
> pgp: 86574ACA Georg C. F. Greve

Gunnar Wrobel
Developer, Kolab Systems AG

e: wrobel at
t: +49 700 6245 0000

pgp: 9703 43BE

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