[tz] What is LMTZ?
guy at alum.mit.edu
Wed Sep 18 23:37:48 UTC 2013
On Sep 18, 2013, at 2:48 PM, Lester Caine <lester at lsces.co.uk> wrote:
> Yes that is all I'm trying to get at. but the point is that it is simply an agreed time for a location which may change as a location uses one or other 'standard'?
A given set of locations may well change what offset from GMT/UTC it uses; those are recorded in the Zone lines for the tzdb zone corresponding to that set of locations.
> And that all this needs is a date and an offset.
A set of dates/times and offsets, to handle the changes.
> At some point in the future we may also need a 'clock' but I suspect that will not be so well documented?
If by "need a 'clock' but I suspect that will not be so well documented" you mean that the changes occur at a particular time on a particular date, whether the particular time is well documented or not, Zone lines already handle that; see, for example, the Zone lines for America/New_York:
Zone America/New_York -4:56:02 - LMT 1883 Nov 18 12:03:58
-5:00 US E%sT 1920
-5:00 NYC E%sT 1942
-5:00 US E%sT 1946
-5:00 NYC E%sT 1967
-5:00 US E%sT
which specify that the America/New_York tzdb zone switched from local time to Eastern Standard time, following standard US "seasonal time shift" rules (which didn't have seasonal time shifts until 1918) on 1883-11-18 12:03:58.
> Lets keep the term 'timezone' for the more complex rules involving all the annual changes which is the only time 'rules' are required?
Unfortunately, at least in the US (and possibly other countries), the term "time zone" is used to refer to regions with a given base standard time; that form of time zone may include multiple subregions with *different* "seasonal time shift" rules. For example, the Mountain Standard Time zone includes Colorado:
# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER
Rule Denver 1920 1921 - Mar lastSun 2:00 1:00 D
Rule Denver 1920 only - Oct lastSun 2:00 0 S
Rule Denver 1921 only - May 22 2:00 0 S
Rule Denver 1965 1966 - Apr lastSun 2:00 1:00 D
Rule Denver 1965 1966 - Oct lastSun 2:00 0 S
# Zone NAME GMTOFF RULES FORMAT [UNTIL]
Zone America/Denver -6:59:56 - LMT 1883 Nov 18 12:00:04
-7:00 US M%sT 1920
-7:00 Denver M%sT 1942
-7:00 US M%sT 1946
-7:00 Denver M%sT 1967
-7:00 US M%sT
Zone America/Phoenix -7:28:18 - LMT 1883 Nov 18 11:31:42
-7:00 US M%sT 1944 Jan 1 00:01
-7:00 - MST 1944 Apr 1 00:01
-7:00 US M%sT 1944 Oct 1 00:01
-7:00 - MST 1967
-7:00 US M%sT 1968 Mar 21
-7:00 - MST
but, as that shows, Colorado currently largely follows the US "daylight savings time" rules and Arizona doesn't.
So I prefer terms such as "tzdb zone" to refer to those things that have tzids. I'm not wedded to *that* term, but there should be *some* term other than "time zone" for that.
The tzdb currently does not have a concept of "time zone" in the sense of a region with a specified base time offset; are you suggesting that it add one?
> Does that simplify the number of timezones problem?
I don't think so. The "number of timezones" problem, if by that you mean the problem that recording the times that particular regions adopted standard time in the tzdb would, as claimed, require thousands of new tzids, would not be solved by terminology - you're going to require thousands of new tzids no matter what.
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