[tz] Hong Kong time 1941

Paul Eggert eggert at cs.ucla.edu
Sun Nov 18 20:04:41 UTC 2018

Michael H Deckers wrote:

>      Decree No 204 of 1904 for Macau says that the switch is
>      done to keep Macau time equal to the time in "Cantão" (Guangdong).

Ah, OK, thanks, I didn't see that.

>      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.

That would partly depend on whether the "full day" is the old or the new one. 
Currently tzdb lists for the Macau transition just "1904 Oct 30" which has the 
effect of Macau switching at the start of the old day, i.e., at 16:25:50 UT. 
(Like you I am using modern UT here, not the pre-1925 astronomical GMT.) If the 
intent was for Macau to switch at the start of the new day, it should indeed 
switch at 16:00 UT. However, Hong Kong switched at 17:00 UT so if the intent was 
for Macau to keep in lock-step with Hong Kong, the Macau transition should be at 
17:00 UT.

If we're lucky, there is documentation (most likely in Portuguese) containing 
details like what we saw for Hong Kong: a proclamation stating the time of 
change, or even records from the Portuguese Navy logging the change (and even 
logging error estimates, wow!). If we're not lucky we'll have to guess.

More information about the tz mailing list