[tz] EU Public Consultation on summertime arrangements

Paul Eggert eggert at cs.ucla.edu
Sun Jul 8 05:27:22 UTC 2018

John Wilcock wrote:
> https://ec.europa.eu/info/consultations/2018-summertime-arrangements_en

Thanks for the heads-up. A draft comment to the EU is attached.
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I am commenting as coordinator of the Time Zone Database hosted by the Internet Assigned Numbers Authority[1]. This database is used by most of the world's cell phones and many of its computers to track time zones and daylight saving time.

Although the simplest thing for computing applications would to keep the EU rules unchanged, it would not be much of a problem to accommodate the abolition of daylight saving time changes in the EU, as has already been done in Russia and other countries. However, if such a change is done, two technical points should be considered.

First, if the EU rules are changed, there should be at least a year's notice before the announced changes take effect. Otherwise there is a good chance that some computer-based clocks will operate incorrectly after the change, due to delays in propagating updates to software and data. The shorter the notice, the more likely clock problems will arise. For more about this, please see "Changes to the tz database"[2] and "On the Timing of Time Zone Changes"[3].

Second, if the EU decides to stop changing clocks twice a year, the resulting time zones should be considered standard time, not daylight saving time, to avoid conflicts with existing computer standards in widespread use. The notion of permanent daylight saving time is not supported by the POSIX.1-2017 standard[4], which covers most of the computing servers in the European Union. Although many POSIX-based systems do support permanent DST, the support is not currently required by the POSIX standard, and if the European Union elected to go to permanent daylight saving, there's a good chance the Time Zone Database would model it as permanent standard time: for example, it would model permanent Central European Summer Time (CEST, UTC +02) not as daylight saving time, but as either permanent Eastern European Time (EET, UTC +02), or as a standard time whose name happened to end "Summer Time". Although this may seem like a small point as the UTC offsets of CEST and EET are identical, having multiple names for the same permanent time zone would undoubtedly confuse users and operators of computers, cell phones, and the like. For this reason, if the twice-yearly clock change is abolished, the EU should simply move time zone boundaries rather than proclaim "permanent summer time" in some areas.


1. Time Zone Database, Internet Assigned Numbers Authority. <https://www.iana.org/time-zones>

2. Changes to the tz database. <https://data.iana.org/time-zones/tz-link.html#changes>

3. Johnson M. On the Timing of Time Zone Changes. 2016-04-23. <https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/>

4. The Open Group Base Specifications Issue 7, 2018 edition, IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008). See section 8.3 "Other Environment Variables", under the variable "TZ" <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03>.

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