`startsAtLabel` asked for `hour12: true` on `en-GB`. That is not the same
request as a 12-hour clock, and it does not survive a Node upgrade: for a
locale whose default cycle is h23, Node 20 resolves `hour12: true` to h11,
whose hours run 0-11, so midnight renders "0:00 am". Node 22 and later
resolve it to h12 and it renders "12:00 am". Same ICU on both sides, so it
is V8's ECMA-402 behaviour rather than locale data.
The image ships node:20-alpine and CI runs Node 20, while a dev machine is
newer -- which is how this rendered correctly in front of everyone who wrote
it and wrongly for every real recipient. An event mail announcing a midnight
start said "0:00 am" while the schedule editor beside it said "12:00 AM":
one instant, two spellings, which is the exact contradiction the option was
added to prevent.
`hourCycle: 'h12'` is the request that means what was meant. `recurrence.js`
already states the mirror-image rule for `h23`, and every other formatter in
this repo and in module-uo uses `hourCycle`; there is no `hour12` left here.
This is the one test that has been red on every events PR since #192, and
the only one -- each of those runs reported `# fail 1`. Verified by running
the suite under node:20-alpine, where the test fails without this change and
2152 tests pass with it; on Node 22+ it passes either way, so the test's
comment now says that a green run on a dev machine is not evidence.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016wDDVXWMDz82WqE1i969r4
wtclaude
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