[frahlg]

Problem

A tester set a car's goal to 100 % by 17:00 on Sunday. The iPhone app showed "100 % by 17:00 on Sun" and, under it, "Next: tomorrow 18:00".

The box plans a goal's clock (ready_by_min, time_of_day_min) in time.Local. When time.Local is UTC, 17:00 is 17:00 UTC, which is 18:00 in Stockholm in winter. Core's DST handling is correct. The app hid the gap: it read a box zone of "UTC" as "the box names no zone" and showed the goal on the phone's clock. It did that because Core also says UTC when it is not:

• Debian links /etc/localtime to Etc/UTC. A container that mounts the host's file (-v /etc/localtime:/etc/localtime:ro) overwrites Etc/UTC itself, so the link, /etc/timezone and the file all say Etc/UTC while Go runs on the host's rules. This is the open P2 on fix(core): name the box's time zone instead of "Local" #1585.
• A copied /etc/localtime with no link and no /etc/timezone fell back to UTC.

So clients cannot trust "UTC" today, and the app's guess is right only by luck.

Change

localzone.Name() returns a name only when that zone has time.Local's rules:

• Each candidate (TZ, the link, /etc/timezone) must show the same UTC offset as time.Local at every instant from 1970 to 2038.
• A UTC name (UTC, Etc/UTC, GMT…) also needs a clock that is UTC by Go's built-in UTC. That catches the overwritten Etc/UTC.
• A copied /etc/localtime is named by the regular Area/City file under the zoneinfo tree with the same bytes, else the first with the same rules. Links and posix/right are skipped.
• An unknown TZ now gives UTC, because Go runs on UTC then. Before, it named the host link's zone.

Paired with srcfl/ftw-app#136. That PR makes the app trust "UTC" from the box and say the goal's time is UTC.

Evidence

• New tests in go/internal/localzone. TestAMountOverTheEtcUTCLinkIsNotUTC fails without the UTC check (zone = "Etc/UTC", want Europe/Stockholm). The copied-file tests fail without the zoneinfo match (zone = "UTC").

• A real run in debian:trixie-slim with tzdata (linux/arm64 under Docker Desktop), printing the name and 16:00Z on the local clock:

  Container	Before	After
  no zone	Etc/UTC, 16:00 UTC	Etc/UTC, 16:00 UTC
  host's Stockholm file mounted	Etc/UTC, 17:00 CET	Europe/Berlin, 17:00 CET
  TZ=Europe/Stockholm	Europe/Stockholm	Europe/Stockholm
  Debian's Stockholm copied as a file	UTC, 17:00 CET	Europe/Stockholm, 17:00 CET

  The mounted file came from macOS's tzdata, so its bytes differ; it is named by rules, and Berlin is the first zone with Stockholm's rules since 1980. The tree walk took about 20 ms, once per process.

• make verify passes locally.

Not in this PR

• The shipped compose files pass no zone, so a Docker box runs on UTC. Now the box says so and the app shows it, but the owner still has to set TZ for goals on their own clock.
• A copied file on a host with no zoneinfo tree still gives UTC.

🤖 Generated with Claude Code

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Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-31T15:44:44.112178Z	f77294e	PR opened

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