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

[Paul Eggert <eggert@cs.ucla.edu>]

On 4/13/22 07:40, Max Nikulin wrote:
> 
> I do not see a way to get 23:30 EAT +0300.

Are you asking for a function F where you say, "I want to give F a 
possibly-ambiguous decoded local time D, and for F to return all 
timestamps that map to D"? If so, encode-time doesn't do that, because 
the underlying C API (namely, mktime) doesn't do that. All mktime and 
encode-time do is give you *one* timestamp that maps to D; it won't give 
you any other timestamps.

If you're worried about possibly-ambiguous decoded local times, you 
could probe (say) one day before and one day after encode-time's result 
to see if the UTC offset changes, and let that guide you to find other 
possible timestamps that map to the decoded time. Although this is just 
a heuristic it should be good enough.

I doubt whether you need to do that, though. Code that is not careful 
about local time offsets doesn't care how ambiguous decoded times are 
resolved. And code that does care should record UTC offsets anyway, and 
you can use those offsets to disambiguate the decoded times. Something 
like this, say:

  (defun encode-and-format-time (time tz)
    (let ((etime (encode-time (parse-time-string time))))
      (format-time-string "%F %T %Z %z" etime tz)))

With this definition, (encode-and-format-time "2021-01-31 23:30:00 
+0300" "Africa/Juba") yields "2021-01-31 23:30:00 EAT +0300", which is 
the timestamp you want.


> `encode-time' should only accept time zone as time offset and should not allow default or named value that may be ambiguous.

If we're talking about Org's encode-time substitute, you can of course 
do what you like. But Emacs encode-time has supported ambiguous 
timestamps for some time and I expect it's used by apps that don't care 
how ambiguous decoded times are resolved, which means we shouldn't 
remove that support without having a very good reason.


> should be possible to provide hints to `encode-time' to get deterministic behavior in the case of time transitions

Yes, that feature is already there. The hint is the UTC offset, as 
illustrated above.