[tz] Java & Rearguard

Guy Harris guy at alum.mit.edu
Fri May 31 22:45:42 UTC 2019

On May 31, 2019, at 9:49 AM, Guy Harris <guy at alum.mit.edu> wrote:

> On May 31, 2019, at 6:52 AM, Brandon Smith <smith.b78987 at gmail.com> wrote:
>> So what is the correct definition of DST or is there one?  If I review the sources linked from the IANA website itself [1], I again see the idea that DST is the process of advancing clocks forward [2] (page #2 and following).  Again, referencing timeanddate.com for Ireland [3] it shows a +1 hour (DST Start) in the Summer (i.e. GMT->IST) and -1 hour (DST End) in the winter (i.e. IST->GMT).  This seems to align with the idea that DST on is the period in which clocks were advanced forward.  I thought part of the broader issue and discussion here was that the tm_isdst flag is now true in winter for Ireland indicating they observe DST on in the winter and not the summer per this prior tz thread [4].
> The definition of "daylight savings time" is up to dictionaries, convention, etc..  It generally appears to mean "the time used during some part of spring and summer, in order to give more hours of daylight in the evening and fewer hours of daylight in the morning".
> "tm_isdst" was originally a UNIXism, so if there's anything that defines what *it* means, it's currently the Single UNIX Specification.  What the Single UNIX Specification page for <time.h>:
> 	https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/time.h.html
> says is
> 	The value of tm_isdst shall be positive if Daylight Savings Time is in effect, 0 if Daylight Savings Time is not in effect, and negative if the information is not available.

On the other hand, the Environment Variables section of the Single UNIX Specification:


says, of the TZ environment variable:

	The expanded format (for all TZ s whose value does not have a <colon> as the first character) is as follows:



	std and dst
		Indicate no less than three, nor more than {TZNAME_MAX}, bytes that are the designation for the standard (std) or the alternative (dst -such as Daylight Savings Time) timezone. Only std is required; if dst is missing, then the alternative time does not apply in this locale.

suggesting that there's "standard time" and "alternative time"; if "standard time" is "standard time as specified by law", then, in the Republic of Ireland, "alternative time" is what's observed in the autumn and winter, when the clocks are turned backwards from standard time.

Perhaps the Open Group should be asked to clarify this.

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