Re: bug#54764: encode-time: make DST and TIMEZONE fields of the list argument optional ones

[Paul Eggert]

On 4/20/22 09:56, Max Nikulin wrote:

> A typo: double "a".

Thanks, fixed in the attached which I installed in master.


> If I get your idea correctly then "January, 31" + "1 month" should be 
> more impressive as impossible date.

Thanks, good idea; also in the attached patch.


> I can not figure out which elisp function can help to 
> determine wall time for Aug 1 start of day in Cairo:
>
> Africa/Cairo  Thu Jul 31 21:59:59 2014 UT = Thu Jul 31 23:59:59 2014 EET 
> isdst=0 gmtoff=7200
> Africa/Cairo  Thu Jul 31 22:00:00 2014 UT = Fri Aug  1 01:00:00 2014 
> EEST isdst=1 gmtoff=10800
>
> input: 2014-08-01 Africa/Cairo
> (timezone may be implicit as the system one)
> expected output: 01:00:00

Given mktime's limitations there's no trivial way to do this for 
arbitrary timestamps, since 00:00 doesn't exist in Cairo that day. 
Worse, in some locations near the International Date Line entire days do 
not exist, because at 00:00 they advanced the clocks forward 24 hours in 
order to move the date line.

It sounds like you're asking for a function that, given a date, yields 
the first broken-down timestamp on or after 00:00 of that date. For 
something like that, I'd use encode-time on 00:00 of that date to get a 
timestamp T, and then use time-add and decode-time to decode T-86400 
seconds, T, and T+86400 seconds, and if the decoded times all look fine 
then return (decode-time T). If not (i.e., their UTC offsets differ, or 
T's decoded time is not 00:00 on the correct date) I'd use binary search 
to find discontinuities between T-86400 and T+86400 and look next to 
those discontinuities to find timestamps closer to what you want.

Of course this is not ideal - but it's similar to what many mktime 
implementations do internally, and it's also similar to what Emacs's 
cal-dst already does (maybe you can look there for ideas), so you'd be 
in good company.

0001-More-encode-time-pitfall-doc-fixes.patch
Description: Text Data