[tz] isdst bug Europe/Dublin (tzdb-2019c)

Stefan Rueger stefan.rueger at theblueorange.space
Wed Dec 11 19:23:27 UTC 2019

On Tue, Dec 10, 2019 at 23:43, Guy Harris wrote:
| I.e., it *is* DST, except that the "S" stands for "spending" rather than "saving". :-)

:-) It's mostly a matter of documentation and expectation. I for one have been surprised by the recent change of interpretation to the extent that I thought this is a bug - despite having read the documentation to a reasonable depth in 2017. I've changed my programme to no longer use isdst (or rely on what I thought isdst meant).

Can I suggest to make explicit in zdump's manpage (and other places) that isdst==1 doesn't necessarily mean clocks going forward. I believe that daylight saving time is widely understood to be synonymous with summer time: https://en.wikipedia.org/wiki/Daylight_saving_time Again, a simple case of noting in the documentation that this is not the intended interpretation here.



I feel there were missed opportunities, too, when developing standards: Rather than foregrounding time difference to GMT and isdst (1 for DXT "daylight changed time", 0 for normal and -1 for unknown) one might have foregrounded the difference to what the region considers its normal time zone and the current offset owing to DXT, if any. Using the current interpretation the DXT offset would be -1 for Ireland and 1 for the UK (while Antarctica/Troll would be 2 and Lord Howe Island 0.5). That way the difference to GMT would be the sum of slow changing (geographic timezone) and potentially annually changing (DXT) offsets. In my mind this would have been more informative and easier to understand than isdst.

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