libc/time: Fix POSIX timezone string parsing

[Zepp-Hanzj]

Summary

• fall back to parsing a POSIX TZ string when loading a zoneinfo file fails
• keep the leading-colon form as file-only, as required by the existing semantics

This addresses the POSIX timezone parsing part of #19446. The reported ROMFS
archive build error is not changed here because it did not reproduce on the
current master branch.

Root cause

The conditions around tzparse() were inverted. The fallback was attempted
only for names beginning with :, and a parse failure was treated as success.
As a result, ordinary POSIX TZ strings such as ART3 and JST-9 fell back to
UTC instead of applying their offsets.

Validation

Tested with sim:nsh using:

CONFIG_LIBC_LOCALTIME=y
CONFIG_LIBC_TZDIR="/etc/zoneinfo"
# CONFIG_LIBC_ZONEINFO is not set

The simulator was rebuilt from make distclean before both runs.

Observed before this change:

TZ=UTC0   08:03:19
TZ=ART3   08:03:36
TZ=JST-9  08:03:56
TZ=:ART3  08:04:06

Observed after this change:

TZ=UTC0   08:09:52
TZ=ART3   05:10:05
TZ=JST-9  17:10:18
TZ=:ART3  08:10:24

ART3 now applies UTC-3, JST-9 applies UTC+9, and :ART3 remains
file-only and falls back to UTC when that file is unavailable.

Additional checks:

• make -j$(nproc)
• tools/checkpatch.sh -f libs/libc/time/lib_localtime.c

[Zepp-Hanzj]

nuttx-tz-after.log
nuttx-tz-before.log
test log here

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb

[acassis]

@Zepp-Hanzj did you test with https://nuttx.apache.org/docs/latest/components/libs/libc/zoneinfo.html ?

[Zepp-Hanzj]

@Zepp-Hanzj did you test with https://nuttx.apache.org/docs/latest/components/libs/libc/zoneinfo.html ?

version.log
build.log
config-check.log
context.log
runtime.log

My previous validation did not enable CONFIG_LIBC_ZONEINFO. I have now retested the current PR with the documented sim:nsh ROMFS configuration:
CONFIG_BOARD_LATE_INITIALIZE=y
CONFIG_LIBC_LOCALTIME=y
CONFIG_LIBC_TZDIR="/share/zoneinfo"
CONFIG_LIBC_TZ_MAX_TIMES=370
CONFIG_LIBC_TZ_MAX_TYPES=20
CONFIG_LIBC_ZONEINFO=y
CONFIG_LIBC_ZONEINFO_ROMFS=y
Both make context and the full build succeeded. The zoneinfo ROMFS was mounted successfully at /share/zoneinfo.
With the same runtime timestamp:
UTC0 2026-07-25T00:38:38
US/Mountain 2026-07-24T18:38:38
:US/Mountain 2026-07-24T18:38:38
ART3 2026-07-24T21:38:38
This confirms that file-backed zoneinfo, including the leading-colon file-only form, still works correctly, while the POSIX fallback now handles ART3 as UTC-3.