Timezone Europe/Dublin has DST flag set in winter

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Environment

SUSE Linux Enterprise 12 SP3 since timezone-2018e
SUSE Linux Enterprise 12 SP4 and later
SUSE Linux Enterprise 15 since timezone-2018e
SUSE Linux Enterprise 15 SP1 and later

Situation

The timezone information for Europe/Dublin has the daylight savings flag (tm_isdst) set in winter.

Cause

The timezone of the Republic of Ireland is defined differently than that of the United Kingdom of Great Britain and Northern Ireland or of other parts of Europe. In summer, the timezone is Irish Standard Time (IST, UTC+1), in winter it is Greenwich Mean Time (UTC+0). Legally, this makes the Republic of Ireland observing a reverse daylight savings time: winter is special, summer is standard. This is reflected by the IANA timezone definitions, as implemented by the GNU Glibc:

# zdump -c 2023,2024 -v Europe/Dublin | grep 2023
Europe/Dublin  Sun Mar 26 00:59:59 2023 UT = Sun Mar 26 00:59:59 2023 GMT isdst=1 gmtoff=0
Europe/Dublin  Sun Mar 26 01:00:00 2023 UT = Sun Mar 26 02:00:00 2023 IST isdst=0 gmtoff=3600
Europe/Dublin  Sun Oct 29 00:59:59 2023 UT = Sun Oct 29 01:59:59 2023 IST isdst=0 gmtoff=3600
Europe/Dublin  Sun Oct 29 01:00:00 2023 UT = Sun Oct 29 01:00:00 2023 GMT isdst=1 gmtoff=0

Compare with:

# zdump -c 2023,2024 -v Europe/Belfast | grep 2023
Europe/Belfast  Sun Mar 26 00:59:59 2023 UT = Sun Mar 26 00:59:59 2023 GMT isdst=0 gmtoff=0
Europe/Belfast  Sun Mar 26 01:00:00 2023 UT = Sun Mar 26 02:00:00 2023 BST isdst=1 gmtoff=3600
Europe/Belfast  Sun Oct 29 00:59:59 2023 UT = Sun Oct 29 01:59:59 2023 BST isdst=1 gmtoff=3600
Europe/Belfast  Sun Oct 29 01:00:00 2023 UT = Sun Oct 29 01:00:00 2023 GMT isdst=0 gmtoff=0

Note that the offset to UTC (tm_gmtoff) is the same at all times in both Dublin and Belfast.

There is another, subtle difference regarding the global variable timezone(3): it is -3600 for
Europe/Dublin (but, as expected, 0 for Europe/Belfast).

According to https://www.gnu.org/software/libc/manual/html_node/Time-Zone-Functions.html , long timezone "contains the difference between UTC and the latest local standard time, in seconds west of UTC. For example, in the U.S. Eastern time zone, the value is 5*60*60. Unlike the tm_gmtoff member of the broken-down time structure, this value is not adjusted for daylight saving, and its sign is reversed. In GNU programs it is better to use tm_gmtoff, since it contains the correct offset even when it is not the latest one."

Additional Information

For more information on the time in the Republic of Ireland refer to https://en.wikipedia.org/wiki/Time_in_the_Republic_of_Ireland , for a discussion about the IANA timezone definition refer to the mailing list thread starting at https://mm.icann.org/pipermail/tz/2017-December/025682.html

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