[tz] www.timeanddate.com provides different data

[Guy Harris]

On Nov 1, 2016, at 5:26 AM, Gintare Statkute <g.statkute at gmail.com> wrote:

> On this website i find the following data:
> https://www.timeanddate.com/time/zone/uk/london
> 
> 2015Sun, 29 Mar, 01:00GMT → BST+1 hour (DST start)UTC+1h 
> Sun, 25 Oct, 02:00BST → GMT-1 hour (DST end)UTC
> 2016 Sun, 27 Mar, 01:00GMT → BST+1 hour (DST start)UTC+1h 
> Sun, 30 Oct, 02:00BST → GMT-1 hour (DST end)UTC
> 2017 Sun, 26 Mar, 01:00GMT → BST+1 hour (DST start)UTC+1h 
> Sun, 29 Oct, 02:00BST → GMT-1 hour (DST end)UTC
> 2018 Sun, 25 Mar, 01:00GMT → BST+1 hour (DST start)UTC+1h 
> Sun, 28 Oct, 02:00BST → GMT-1 hour (DST end)UTC
> 2019 Sun, 31 Mar, 01:00GMT → BST+1 hour (DST start)UTC+1h 
> Sun, 27 Oct, 02:00BST → GMT-1 hour (DST end)UTC
> 
> As you can see, the Europe/London time change rule is different every year .

2015: the UK changes from standard time to summer time on the last Sunday in March.

2016: the UK changes from standard time to summer time on the last Sunday in March.

2017: the UK changes from standard time to summer time on the last Sunday in March.

2018: the UK changes from standard time to summer time on the last Sunday in March.

2019: the UK changes from standard time to summer time on the last Sunday in March.

That's not different every year.  The date of the last Sunday in March is different every year, but that's just the way the Gregorian calendar works.

> But i can not find this information on yor data distribution.
> https://www.iana.org/time-zones
> 
> For europe, london you give:
> # Zone    NAME        GMTOFF    RULES    FORMAT    [UNTIL]
> Zone    Europe/London    -0:01:15 -    LMT    1847 Dec  1  0:00s
>              0:00    GB-Eire    %s    1968 Oct 27
>              1:00    -    BST    1971 Oct 31  2:00u
>              0:00    GB-Eire    %s    1996
>              0:00    EU    GMT/BST

That says that, from 1996 on, they used the "EU" rules.  Those should also be in the "europe" file; the 2016g file says:

	# Rule	NAME	FROM	TO	TYPE	IN	ON	AT	SAVE	LETTER/S
	Rule	EU	1977	1980	-	Apr	Sun>=1	 1:00u	1:00	S
	Rule	EU	1977	only	-	Sep	lastSun	 1:00u	0	-
	Rule	EU	1978	only	-	Oct	 1	 1:00u	0	-
	Rule	EU	1979	1995	-	Sep	lastSun	 1:00u	0	-
	Rule	EU	1981	max	-	Mar	lastSun	 1:00u	1:00	S
	Rule	EU	1996	max	-	Oct	lastSun	 1:00u	0	-

So, from 1981 on, the "EU" rules specify that the time changes to summer time at 1:00 UTC on the last Sunday in March.

As Paul Eggert indicated, the "zic" man page describes how those files work.