[tz] www.timeanddate.com provides different data

[Matt Johnson]

You may also find the following article to be useful:


http://www.iana.org/time-zones/repository/tz-how-to.html




________________________________
From: tz-bounces at iana.org <tz-bounces at iana.org> on behalf of Guy Harris <guy at alum.mit.edu>
Sent: Tuesday, November 1, 2016 2:20 PM
To: Gintare Statkute
Cc: tz at iana.org
Subject: Re: [tz] www.timeanddate.com provides different data

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
Daylight saving time dates for United Kingdom - England ...<https://www.timeanddate.com/time/zone/uk/london>
www.timeanddate.com
Historic, present and future dates for daylight saving time and clock changes. Time changes between years 2015 and 2019 in United Kingdom - England - London are ...



>
> 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
IANA - Time Zone Database<https://www.iana.org/time-zones>
www.iana.org
Time Zone Database. The Time Zone Database (often called tz or zoneinfo) contains code and data that represent the history of local time for many representative ...



>
> 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.
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