Re: tzset: add native Windows workaround

[Paul Eggert]

Bruno Haible wrote:

> Only for Cygwin, an empty or absent TZ environment variable means GMT.

That's weird; I don't know of any other system that does that. In many systems, 
an empty but set TZ means UTC0 without leap seconds; but an unset TZ typically 
means "ask the file system for the time zone", e.g., /etc/localtime or something 
like that. Perhaps the Cygwin folks could be talked into changing an unset TZ to 
mean "ask the Windows Control Panel".


> The full set of TZ values understood by MSVC CRT is not documented. (Maybe
> there are more valid strings than those that fit your regexp?)

Maybe there are. However, I think the ERE captures all the values that Microsoft 
documents, and this should be good enough.

> How about this revised comment?

> +     There are two possible kinds of such values:
> +       - Traditional US time zone names, e.g. "PST8PDT",
> +       - tzdata time zone names, based on geography.  They contain one
> +         or more slashes.

As a point of terminology, "PST8PDT" is as much of a tzdata time zone name as 
"America/Los_Angeles" is. They are both implemented by consulting a file by that 
name. And some of the slashless tzdata names are based on geography, e.g., 
"Singapore", "GB-Eire" (these are mostly present for backward compatibility, but 
some people still use them). Perhaps it would be better to summarize things by 
wording the comment something like this:


TZ values are of two kinds:

- Values supported by the Microsoft CRT, e.g., "PST+8PDT". See 
<https://msdn.microsoft.com/en-us/library/90s5c885.aspx>. The documented values 
of this form are matched by the POSIX extended regular expression 
"^[A-Za-z]{3}[-+]?[01]?[0-9](:[0-5][0-9](:[0-5][0-9])?)?([A-Za-z]{3})?$".

- Values supported by Cygwin, e.g., "America/Los_Angeles". Typically, each of 
these values corresponds to the name of a file installed somewhere on the 
system. However, some of these values are analyzed programmatically based on 
rules specified by POSIX, e.g., "PST8PDT,M3.2.0,M11.1.0"; see 
<http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03>.

The two kinds of TZ values overlap, e.g., "PST8PDT" is valid in both 
implementations.  However, most TZ values supported by Cygwin do not work with 
the Microsoft CRT, which silently uses UTC when given such values. In practice 
most of these troublesome TZ values contain '/', and no TZ value supported by 
the Microsoft CRT contains '/', so as a heuristic neutralize any TZ value 
containing '/'. For the Microsoft CRT, an absent or empty TZ means the time zone 
that the user has set in the Windows Control Panel.



> +     neutralize it.  For the Microsoft CRT, an absent of empty TZ means

absent of -> absent or