[Bug time/31144] mktime: returns clock for UTC with isdst=1

["eggert at cs dot ucla.edu" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31144

eggert at cs dot ucla.edu changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |NOTABUG

--- Comment #2 from eggert at cs dot ucla.edu ---
The C and POSIX standards do not specify what mktime should do with examples
like this, so this is not a conformance issue. As Andreas mentioned, tm_isdst
is intended as a hint for ambiguous timestamps; unfortunately this does not
suffice in general for TZDB Zones, e.g., 1971-10-31 02:30 when
TZ='Europe/London' because at 03:00 standard time moved back by an hour in
London so there are two distinct matching timestamps both with tm_isdst==0, one
with time_t==57720600 and one with time_t==57724200. So the whole area is a bit
of an ill-thought-out mess.

glibc currently supports the idea that if you want to find a time that's 1 year
from now you can call localtime, add 1 to tm_year, and then call mktime on the
result, and you'll get a timestamp that's 1 year later or thereabouts (the
concept of "one year later" is not well-defined). This idea didn't work with
older glibc (and I guess FreeBSD libc?) if DST is in use now but not 1 year
from now (or vice versa) because in those cases older glibc mktime simply
failed.

In practice the new glibc behavior seems to be more useful than the old one.
The old behavior was changed in response to a real-world use case
<https://bugs.gnu.org/48085> whereas the Erlang/OTP issue is an artificial test
case which is a weaker argument.

With that in mind I'm going to resolve this as NOTABUG. The Erlang/OTP test
case is wrong because it's making assumptions about mktime that are not present
in the relative standards

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