[tz] Did Greenland abolish daylight saving from 2024 on?

Guy Harris gharris at sonic.net
Fri Nov 17 19:34:12 UTC 2023

On Nov 17, 2023, at 8:35 AM, Paul Eggert <eggert at cs.ucla.edu> wrote:

> On 2023-11-17 01:00, Guy Harris wrote:
>> So does POSIX's use of "Daylight Savings Time" mean that "daylight saving time" could either be a time when the clock is turned forward from standard time *or* could *be* standard time, with the clocks being turned *backwards* from standard time for the winter?
>> Or does it really mean that "daylight saving time" can refer to a period in which the clock is turned *backwards* from standard time?
> The latter is true. (I don't fully understand what you mean by the former, or why it differs from the latter.)

The former would mean that "Daylight Savings Time", in POSIX, is always a time when clocks are turned forward, i.e. is always "summer time", even if that means that "standard time" and "Daylight Savings Time" are in effect at the same time.

The latter would mean that "Daylight Savings Time", in POSIX, could either be a time when clocks are turned forward or a time when clocks are turned backward, i.e. it could either be "summer time" or "winter time".

Do they explicitly define "Daylight Savings Time" in this fashion in some place?

8.3 "Other Environment Variables" says, in its description of TZ:

	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.

which speaks of "standard" and "alternative" timezones.

They don't state what "standard" means; it might be useful if POSIX were to indicate that this term refers to something such as the timekeeping specified as a "standard" by law.

I like the term "alternative", as it's not the problematic-in-any-ways "daylight saving time" and "daylight savings time", and as it does not commit to the alternative to always be "summer time", allowing it to be "winter time".  That merely speaks of "Daylight Savings Time" as an *example* of an "alternative".

The only way in which I could see it being read as assigning a meaning to "Daylight Savings Time" is indirectly, by speaking of "std" and "dst" as fields in the TZ environment variable setting (rather than speaking of "std" and "alt"), as "dst" suggests that it refers to "Daylight Savings Time", and thereby suggesting that "Daylight Savings Time" is an alternative to "standard time".

The problem I have with that is that, as far as I know, in the regions where terms of the form "daylight saving[s] time" are used, it exclusively refers to times when clocks are turned forward, i.e. to what other regions call "summer time", so, in the absence of an explicit definition of "Daylight Savings Time", people might assume it refers to clocks-turned-forward-time, in which case "Daylight Savings Time" would not be in effect in Ireland during the winter.

I.e., the problem with

> If you're accustomed to the United States then that's indeed what "daylight saving time" has meant. And given the US's influence it's not surprising that the usage is common elsewhere. However, it's not universal, and POSIX and TZDB support the more-general case.

is that, as far as I know, in those locations where clocks are turned backward from "standard time" in the winter, rather than forward from "standard time" in the summer, that's referred to as "winter time", not "daylight saving time" or any such term, so it's not clear that the use of "daylight saving time" etc. to refer to turning clocks forward from "standard time" in the summer is *not*, in fact, universal.  See, for example:


in which "daylight saving time" is used in the US sense, and


which says both

	The time change in winter, commonly referred to as “Daylight Saving Time” (DST) or “Standard Time,”

which I find difficult to interpret, and both

	When DST begins in the spring, ...


	When DST ends in the winter ...

which uses DST in the US sense.  Is there *any* place where non-computer-nerds and non-time-nerds speak of "daylight saving time" being in effect during the winter?  If not, then perhaps POSIX, to avoid the need to ask questions of its maintainers as to what they mean, should either define what "Daylight Savings Time" means (whether it's intended to mean "summer time" or it's intended to mean "alternative time" as in "not standard time"), *even if that means it doesn't correspond to the US-style usage* (it might not be a *wise* choice to use it in that fashion, as it appears to be used in that fashion even in at least some Irish web sites), or should use "alternative time" and define *that* to mean "not standard time", and perhaps explicitly indicate that this could mean either "summer time" or "winter time".

> Say you're in Ireland and set TZ='IST-1GMT0,M10.5.0,M3.5.0/1'. POSIX says that tm_isdst must be positive when clocks are turned backwards from standard time. This longstanding POSIX requirement is independent of whatever TZDB does.

I.e., POSIX is saying that tm_isdst must be positive during a time that at least some Irish web sites consider not to be Daylight Saving Time.

Or, alternatively, that a value for tm_isdst that's positive could mean that the time in question is not DST, not that it is DST.

More information about the tz mailing list