[tz] Inquiry to NICT about Japanese Summer time in 1948-1952.

Paul Eggert eggert at cs.ucla.edu
Tue Sep 11 17:00:20 UTC 2018

On 9/11/18 12:28 AM, aya sekido wrote:
> According to the law of those days, the last day of summer time was 
> made into 25 hours.
> What came after 12:59 p.m. of the last day was 00:00 a.m. of the next day.
Thank you for looking into it. The Japanese law that we found 
"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。" 
Google translates this 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."

I see two ways to interpret how this law affected timekeeping during the 
September transition:

1. At 24:00 Saturday, people moved their 12-hour clocks back from 12 
o'clock midnight to 11 o'clock P.M. Saturday, thus repeating the 23:00 
Saturday hour, so that Saturday had 25 hours.

2. At 24:00 Saturday people left their clocks alone, and let them 
continue forward from 12 o'clock to 1 o'clock. The day was still 
Saturday, but the time was in the range 24:00 to 25:00. At 25:00 
Saturday, people moved their 12-hour clocks back from 1 o'clock A.M. to 
12 o'clock midnight and Sunday began at 00:00. This method also gives 25 
hours to Saturday, and it is what you are suggesting.

Method (1) would be likely used in the West; however, as I understand it 
method (2) is plausible in Japan (nowadays a Japanese bar might give a 
closing time of "26:00", for example). It is a tricky situation, since 
the law in question was done at the instruction of the Supreme Commander 
of the Allied Powers which may well have ordered method (1). Although 
method (1) is what we currently have in the Internet Time Zone Database 
(tzdb), if the Japanese people generally used method (2) we should try 
to fix tzdb as best we can; unfortunately we cannot do so perfectly, 
since the database format does not let us represent times like "24:59".

Is there a good way to resolve the question? For example, are there 
archived versions of Japanese newspapers from the early 1950s?

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