[C++] Vendored date library does not respect TZDIR environment variable #49351

The vendored Howard Hinnant date library hardcodes /usr/share/zoneinfo as the timezone database path in discover_tz_dir() (cpp/src/arrow/vendored/datetime/tz.cpp). It does not check the TZDIR environment variable, which is the POSIX standard mechanism for overriding this path.

This causes timezone operations to fail on non FHS Linux distributions such as NixOS, where zoneinfo resides under a non standard path like /nix/store/.../share/zoneinfo.

The upstream library also lacks TZDIR support

Reproduction

On NixOS without a TZDIR patch:

import pyarrow as pa

arr = pa.array([1], type=pa.timestamp("s", tz="America/New_York"))
# Fails: discover_tz_dir failed to find zoneinfo

Expected Behavior

Arrow should check TZDIR before falling back to hardcoded paths, consistent with POSIX conventions and tools such as glibc, busybox, and Python's zoneinfo module.

Component(s)

C++