[tz] strftime %s

[Paul Eggert]

On 2024-01-12 00:02, Robert Elz wrote:

>    | If I understand things correctly, it's impossible for an implementation
>    | to conform to both POSIX-2017 and draft next POSIX in this area.
> 
> What do you believe the problem to be?

On reflection I spoke too quickly and these appear to be awkward 
wordings and misinterpretations rather than conformance issues. Still, 
the awkwardnesses deserve discussion.

POSIX 202x/D3 and the latest C2x draft (n3096) can both plausibly (but 
in my view incorrectly) be interpreted to say that strftime cannot look 
at tm_gmtoff or tm_zone, which is obviously wrong for tzcode, as well as 
for glibc, FreeBSD, and I assume other strftime implementations.

To work around this problem, it'd be helpful if draft POSIX (at least) 
were fixed to clearly state the interpretation I proposed earlier today 
<https://mm.icann.org/pipermail/tz/2024-January/033524.html>.

It'd also be helpful if draft POSIX were changed to state the following:

* For %c and %X, strftime can look at tm_gmtoff and tm_zone.

* For %s, strftime can look at tm_gmtoff.

* For %Z, strftime can look at tm_zone.

* For %z, strftime can look at tm_gmtoff and tm_zone. (tm_zone is useful 
when tm_gmtoff == 0, since it helps distinguish +0000 from -0000.)

This would better reflect existing and reasonable practice.


Less importantly, POSIX and C both say that strftime can look at struct 
tm members that aren't needed once you have tm_gmtoff and tm_zone (and 
in some cases aren't needed even if you have those two new members). 
POSIX should make it clear that the implementation need not look at 
these members.

* For %c, %s, %Z and %z, strftime need not look at tm_isdst.

* For %U, strftime need not look at tm_year.

* For %X, strftime need not look at tm_year, tm_yday, tm_mon, tm_mday, 
tm_wday, tm_isdst.

* For %x, strftime need not look at tm_hour, tm_min, tm_sec, tm_gmtoff, 
tm_zone, tm_isdst.


To try to document all this better from tzcode's point of view, I 
installed the attached proposed patch to tzcode's strftime man page.
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