Given that I've already found discrepancies (see "Discrepancies in time zone data interpretation") I'm going to go ahead and hack on this in purely pragmatic (read: short term) ways. I'll create a github repo just for this purpose and dump code in there - this is explicitly with the aim of encouraging a more permanent solution by proving value. Will post another message here when there's something worth looking at - I'll be initially looking at zdump output, Joda Time, standard Java, and Noda Time. Contributions from others for other languages/platforms will be very welcome. Jon On 13 July 2015 at 14:46, Stephen Colebourne <scolebourne@joda.org> wrote:
FWIW, I think such a format would be very useful. Effectively, it is a unit test for others to confirm that they interpret the rules the same way as intended.
It is similar to what I produced when trying to demonstrate the amount of change being caused by apparently "minor" changes to the data: https://github.com/jodastephen/tzdiff/commits/master
Any output of this type should indeed just consist of a simple text file with ISO-8601 format timestamps.
Stephen
Background: I'm the primary developer for Noda Time which consumes the tz data. I'm currently refactoring the code to do this... and I've come across some code (originally ported from Joda Time) which I now understand in terms of what it's doing, but not exactly why.
For a little while now, the Noda Time source repo has included a text dump file, containing a text dump of every transition (up to 2100, at the moment) for every time zone. It looks like this, picking just one example:
Zone: Africa/Maseru LMT: [StartOfTime, 1892-02-07T22:08:00Z) +01:52 (+00) SAST: [1892-02-07T22:08:00Z, 1903-02-28T22:30:00Z) +01:30 (+00) SAST: [1903-02-28T22:30:00Z, 1942-09-20T00:00:00Z) +02 (+00) SAST: [1942-09-20T00:00:00Z, 1943-03-20T23:00:00Z) +03 (+01) SAST: [1943-03-20T23:00:00Z, 1943-09-19T00:00:00Z) +02 (+00) SAST: [1943-09-19T00:00:00Z, 1944-03-18T23:00:00Z) +03 (+01) SAST: [1944-03-18T23:00:00Z, EndOfTime) +02 (+00)
I use this file for confidence when refactoring my time zone handling code - if the new code comes up with the same set of transitions as the old code, it's probably okay. (This is just one line of defence, of course - there are unit tests, though not as many as I'd like.)
It strikes me that having a similar file (I'm not wedded to the format, but it should have all the same information, one way or another) released alongside the main data files would be really handy for all implementors
On 11 July 2015 at 11:35, Jon Skeet <skeet@pobox.com> wrote: -
it would be a good way of validating consistency across multiple platforms, with the release data being canonical. For any platforms which didn't want to actually consume the rules as rules, but just wanted a list of transitions, it could even effectively replace their use of the data.
One other benefit: diffing the dump between two releases would make it clear what had changed in effect, rather than just in terms of rules.
One sticking point is size. The current file for Noda Time is about 4MB, although it zips down to about 300K. Some thoughts around this:
We wouldn't need to distribute it in the same file as the data - just as we have data and code file, there could be a "textdump" file or whatever we'd want to call it. These could be retroactively generated for previous releases, too. As you can see, there's redundancy in the format above, in that it's a list of "zone intervals" (as I call them in Noda Time) rather than a list of transitions - the end of each interval is always the start of the next interval. For zones which settle into an infinite daylight saving pattern, I currently generate from the start of time to 2100 (and then a single zone interval for the end of time as Noda Time understands it; we'd need to work out what form that would take, if any). If we decided that "year of release + 30 years" was enough, that would cut down the size considerably.
Any thoughts? If the feeling is broadly positive, the next step would be to nail down the text format, then find a willing victim/volunteer to write the C code. (You really don't want me writing C...)
Jon