On 9/18/18 1:43 PM, Paul Ganssle wrote:
The law says that the last day of DST had 25 hours instead of 24. The question is whether in practice what happened was that starting 0:00 on Saturday, did people wait 25 hours (until 01:00 on Sunday) and THEN set their clocks back 1 hour, or did they wait 24 hours (until 00:00 on Sunday), and then set the clock back to 23:00?
Yes, that's the nub of the question. If people in Japan generally did the former, we should change tzdb to model the transition as one from 01:00 to 00:00 Sunday; this is not exact but is the best we can do. If people generally did the latter we're OK as-is. Possibly some people did one thing while others did the other, as the American occupiers did not always see eye-to-eye with the Japanese populace and I doubt whether it's entirely a coincidence that Japan stopped observing DST three days after US occupation ended. As we've already mentioned, old Japanese-language newspapers could help resolve this issue.