Change TZDEFRULES and TZDEFRULESTRING to "GMT".

[michaellenaghan]

This is a companion to changing the default TZ value from "GST" to "GMT". The rationale is the same: it’s a less surprising "not defined" default.

For the curious, details follow.

===

TZDEFRULES is defined here and only used here, by localtime_tzparse().

In the original IANA code, the value of TZDEFRULES is "posixrules". "posixrules" was a POSIX fallback mechanism that tried to derive rules for one timezone from another. Foolish mortals! It was declared obsolete in the 2019b IANA release. (For more info about what it tried to do, see man 5 tzfile.)

If TZDEFRULES were set to NULL, localtime_tzparse() would fall back to TZDEFAULT, because of this:

cosmopolitan/libc/time/localtime.c

Lines 382 to 383 in e0c2b91

	if (! name) {
	name = TZDEFAULT;

A recent pull request set TZDEFAULT to "GMT", so in a sense setting TZDEFRULES to NULL is another option. Setting it directly to "GMT", though, does a better job of making the intent obvious: if you can't find any rules, don't apply any rules.

TZDEFULESTRING is defined here and only used here, also in localtime_tzparse().

The value I've used, "GMT0", is the same value gmtload() passes to localtime_tzparse():

cosmopolitan/libc/time/localtime.c

Lines 1372 to 1376 in e0c2b91

	gmtload(struct state *const sp)
	{
	if (localtime_tzload(gmt, sp, true) != 0)
	localtime_tzparse("GMT0", sp, NULL);
	}

The format of the string is defined in Section 8.3 of the POSIX standard, available here.

I'm not sure I really wanted to know this much about how this stuff works. (How much do I really know…?) I'm not sure you really wanted to know this much about how this stuff works either. But here we are.

P.S. All of this is, in some sense, pointless; even redbean-2.2.com falls back to "GMT" if a timezone isn't found:

TZ="xxx" ./redbean-2.2.com -i -e "print(unix.localtime(unix.clock_gettime()))"
2023	7	26	17	32	38	0	3	206	0	GMT

This change really only affects deeper, darker corners of the code. "You're in a maze of twisty little passages…"