On 2018-11-18 02:05, Paul Eggert asked:
How do we know that they switched at the same instant?
Because Decree No 204 of 1904 for Macau says that the switch is done to keep Macau time equal to the time in "Cantão" (Guangdong).
Although (unlike Hong Kong) for Macao we have a copy of the government order (it's cited in tzdb), that order doesn't specify even the date of transition, much less a time of day.
Incorrect. The decree does give the date ("30 do corriente") for the switch and it is quite clear about the time of day of the switch: it states that the new offset applies since Oct 30, and for the full day of Oct 30. As the offset is increased, this implies that the switch happened at the instant when UT was 1904-10-29T16:00. Actually, the switch described in decree 204 is the one done in "Cantão", but it is obviously also intended to be applied in Macau. Michael Deckers.