[tz] 1948-1951 Asia/Tokyo DST information
eggert at cs.ucla.edu
Sat Sep 8 15:49:11 UTC 2018
Phake Nick wrote:
> Not really, it sat 25:00:00 at Saturday would become 00:00:00 on Sunday It
> used the tine from Saturday to refer to transition that occur at Sunday
The cited Japanese law
says, "第二条 四月の第一土曜日の翌日（日曜日）は二十三時間をもつて一日とし、九月の第二土曜日は二十五時間をもつて一日とする",
which Google translates as "Article 2. The day following the first Saturday of
April (Sunday) shall be one day with twenty-three hours and the second Saturday
of September shall be twenty-five hours a day." This law is compatible with what
tzdb specifies, which is that the clock jumps back from 24:00 to 23:00 on
Saturday, which means Saturday indeed has 25 hours, the last of which has
timestamps duplicated from the next-to-last.
As I understand it, you're saying that the practice in Japan (not specified in
the law) was to call these timestamps 24:00 through 24:59 instead. A nice
property of this practice would be that timestamps would not be ambiguous.
During the period in question (1948-1951) it's unlikely that many physical
clocks showed the times 24:00 through 24:59, so the question is whether people
adjusted their 12-hour clocks from 1 o'clock (representing 25:00 Saturday) to 12
o'clock (representing 00:00 Sunday), or from 12 o'clock (representing 24:00
Saturday) to 11 o'clock (representing 23:00 Saturday). Both actions are
consistent with the cited law, and so far we have no evidence what people
actually did. Since tzdb cannot implement what you're suggesting, the answer to
this question won't affect what we put into tzdb proper, though of course it
should affect the commentary.
One other thing. If your suggestion is that the practice was to simply number
the timestamps in a day from 00:00 to the end of the day regardless of the day's
length, then for consistency the timestamps in a spring-forward Sunday (i.e.,
the Sunday after a spring-forward transition) should have been 00:00 through
22:59. That is, in spring people in Japan would have moved their 12-hour clocks
from 11 o'clock to 12 o'clock at 23:00 Sunday. I'm skeptical that this occurred,
as it's typical to make the change Sunday morning in order to minimize
disruption to the workweek starting Monday, so it's far more likely that people
would have moved their clocks from 12 o'clock to 1 o'clock at 00:00 Sunday, as
tzdb has it now. Surely a "just number the timestamps in a day consecutively"
rule cannot be what was used in Japan; if there was a rule about 24:00 through
24:59 it was a special-case rule that appled only to fall-back Saturdays.
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