If the tzdata isn't really intended to be consumed - if we should
really only consume the zic output, and anything else is somewhat questionable, then why distribute the tzdata at all? Why not just distribute the zic output files?
As for DST vs STD time not being relevant in software - while Windows doesn't (mostly) use tzdata, it does allow you to specify a time zone and exclude DST from that. Anyone wanting to mimic that behaviour but using the tz data can only do it if they know the DST component of the overall offset.
I would really like to proceed pragmatically: I have a personal real-world need for validation, and given the discrepancies I've found so far, I think it would be useful to other people as well. I would much rather have an imperfect but widely-used solution that has scope to be improved later than a centithread here but with no practical outcome.
Some observations I hope we can all agree on:
- Some platforms do consume tzdata directly, and expose the STD/DST components of the overall offset, and are likely to continue doing so.
- zic and the reference API do not expose the STD/DST components of the overall offset, and are unlikely to start doing so
- If two implementations agree on overall offsets, it's highly likely they agree on STD/DST components, whether or not that can be verified
- It is desirable that all implementations agree on as much as possible, and this is more likely to happen if it's easy to regularly validate
- It is also informative to be able to easily tell the differences between different data versions (whether released or not)
Now, a few more debatable thoughts:
- Tools:
- I think it would be reasonably logical - and probably fairly simple - to modify zdump to output whatever format we want, assuming we don't intend to include the STD/DST components. I'm not sure how easy it is to get a complete list of all time zones to make zdump dump them all.
- Modifying zic would give us more information to dump, but wouldn't make as much sense logically.
- Writing a brand new tool with the logic of zic but just for dumping would probably involve code duplication, which introduces the possibility of the codebases diverging accidentally.
- Format:
- If we were to use a format such as XML, JSON or YAML to represent the data, it's easier to make it extensible and it's also more easily processed by modern platforms (Java, .NET etc) without fiddly line-based parsing. On the other hand, that sometimes requires extra dependencies - and may well be annoying from C. (I'm aware that zdump and zic are pretty simple to build - and need to be built on a very wide variety of platforms.)
- A line-based format is easier to diff with common tools than a more structured format.
- Writing a tool to convert between formats would be near-trivial if we bear it in mind from the beginning.
- If the format allows the fields included to be specified, it will allow STD/DST-component-aware platforms to compare against each other for differences, even if they can only compare total offsets against zic output
- Despite the contents of my current github repo, I'm certainly not proposing actually using Windows line breaks for the "real" format.
- Process:
- It should be very easy to add a commit hook to github to generate a new dump file per commit, making it really easy to diff any pair (e.g. between two releases, or seeing effect a recent commit had, whether of code or data.)
- While I believe it would be beneficial to ship a dump file alongside code/data releases, with the previous bullet in place, we wouldn't need that to start with. We could view the whole thing as experimental until we're happy with the format etc.
- zic documentation:
- All of this has come from me struggling to implement a tzdata parser which is in line with zic. The man page for zic documents the tz data format, but not in enough detail for a compliant implementation, in my view. I volunteer to at least attempt some more detailed documentation if others feel it's useful. (If no-one else does, I'll probably keep it under the Noda Time documentation anyway, but with a suitable "this is entirely unofficial" warning.)
So, where do we go from here? Does anyone believe this would actually be a bad thing to have? (That might come with the position of "only use zic output".) How are we best to decide the format? If modifying zdump to add an extra flag is deemed an appropriate course of action, do we have any volunteers to do so? I'm happy to host a github hook to publish the dump files at each commit when all the rest of the machinery is in place.
Jon