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January 2021

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zic bug: Wall-clock ordering of transition times
by Tim Parenti Jan. 4, 2021

Jan. 4, 2021

I noticed a bug while incorporating changes to commentary for the Turks and Caicos Islands… https://mm.icann.org/pipermail/tz/2020-December/029609.html Our contemporaneous use of "3:00" in the UNTIL field for the 2018-03-11 change intentionally avoided two clock changes in a row (first from 01:59:59 AST to 01:00:00 EST, then from 01:59:59 EST to 03:00:00 EDT an hour later). https://github.com/eggert/tz/commit/8792d7bed2c684cf842592fd7779645f335a33f6 My recently-added commentary clarifies the reasoning behind this and concludes that the order was likely intended to take effect from 03:00 -04, once the Eastern US had sprung forward, and not at 02:00 -04 as the text suggests. However, I noticed that using "2:00" in the UNTIL field instead doesn't actually result in the expected extra transition: --- to2050.tzs 2020-12-05 14:11:12.000000000 -0500 +++ to2050new.tzs 2020-12-05 14:17:31.000000000 -0500 @@ -5201,7 +5201,7 @@ 2014-03-09 03 -04 EDT 1 2014-11-02 01 -05 EST 2015-03-08 03 -04 AST -2018-03-11 03 -04 EDT 1 +2018-03-11 02 -04 EDT 1 2018-11-04 01 -05 EST 2019-03-10 03 -04 EDT 1 2019-11-03 01 -05 EST It seems that, the 02:00 wall-clock time having already been approached once for the transition out from AST, zic assumes that the transition specified in the US Rule at the same wall-clock time has also taken simultaneous effect in Eastern Time, although in reality it wouldn't yet for another hour. In this case, the wall-clock times of these two transitions are identical, while in actuality the transitions are meant to take effect sequentially. It appears that this bug also manifests in other cases where the strict ordering of transitions does not exactly match the strict ordering of wall-clock times: Trying out "2:30" in the UNTIL field should mean that the Zone transition should happen 30 minutes BEFORE the US Rule would take effect (and thus require two transitions), but by wall-clock time, it would "appear" to be 30 minutes AFTER (fooling zic into thinking only the "latter" Zone transition is needed as the Rule would have "already" taken effect). And, indeed, the results are similarly incorrect here, too: --- to2050.tzs 2020-12-05 14:11:12.000000000 -0500 +++ to2050new.tzs 2020-12-05 14:36:07.000000000 -0500 @@ -5201,7 +5201,7 @@ 2014-03-09 03 -04 EDT 1 2014-11-02 01 -05 EST 2015-03-08 03 -04 AST -2018-03-11 03 -04 EDT 1 +2018-03-11 02:30 -04 EDT 1 2018-11-04 01 -05 EST 2019-03-10 03 -04 EDT 1 2019-11-03 01 -05 EST Certainly an edge case in this particular instance, but given that many historical transitions coincide tightly with transitions in other regions being "joined" or "left", perhaps one worth a further look. -- Tim Parenti

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