[tz] Irish Standard Time vs Irish Summer Time

[Paul Eggert]

On 12/09/2017 06:00 AM, Mark Davis ☕️ wrote:
> I am worried that a significant number of implementations may keel 
> over if handed negative offsets.

So far we've found one specific trivial formatting glitch 
<https://mm.icann.org/pipermail/tz/2017-December/025696.html>, and one 
library <https://mm.icann.org/pipermail/tz/2017-December/025694.html> 
with a debugging assertion about DST offsets that was incorrect. If 
these examples are typical, then there is little to worry about; if 
more-significant problems show up, then the tzdata change may need to be 
reverted.

There is a tension here between trying to support daylight-saving time 
practice, and trying to keep our code and machines running. Although we 
want both objectives, they sometimes compete. Here the gain is small 
(supporting IST as it was intended). If the cost is trivial (a few 
formatting glitches or debug runs fail until software is corrected) then 
the gain is worth the cost; if the cost is large (some user programs 
crash in typical operation) then it's not. So far we've seen only 
trivial costs.

PS. Howard, that debugging assertion change 
<https://github.com/HowardHinnant/date/commit/1eeb4cd6522da8c05ccadf3868a076ba1856e61e> 
still looks dicey. It still forbids 3-hour DST, for example, even though 
POSIX TZ strings and the tzfile format both require support for 3-hour 
DST. POSIX requires support for UT offsets ranging from -24:00 to 
24:59:59, so in theory the difference between DST and STD could be in 
the range -48:59:59 .. 48:59:59. tzdata supports offsets that are way 
wider, for what it's worth.