Re: mktime result may depend on previous calls

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

On 1/18/23 19:00, Max Nikulin via Libc-alpha wrote:

> Paul, thank you for clarification. Should some note be added to the docs?

The actual behavior is reasonably complicated. As a time nerd I'm 
probably the wrong person to ask about whether it should be documented.

> 
> ---- >8 ----
> Notice that in the case of ambiguous local time (around a transition 
> with backward step) multiple mktime calls with the same argument may 
> return both values. Such kind of uncertainty appears if tm_isdst is set 
> to -1 or if local time changes keeping the same DST state.
> ---- 8< ----

The above wording isn't quite right, as it ignores the role of tm_gmtoff 
in mktime. (Did I say it was complicated? :-)


> It even might be used as a non-portable way of disambiguation: just add 
> an extra call of mktime well aside from the backward time step. Too 
> weird to use it in practice though. Unfortunately POSIX does not allow 
> anything like
> 
> https://www.python.org/dev/peps/pep-0495/
> PEP 495 – Local Time Disambiguation (Python)

POSIX does allow that sort of thing as an extension, and glibc mktime 
does so: it disambiguates via tm_gmtoff.