diff --git a/server/src/events/announce.js b/server/src/events/announce.js
index e8d0f76..db52ade 100644
--- a/server/src/events/announce.js
+++ b/server/src/events/announce.js
@@ -67,7 +67,18 @@ function startsAtLabel(at, zone) {
       // Explicit rather than left to the locale, because `en-GB` would otherwise
       // render midnight as "00:00" while the schedule editor beside it writes
       // "12:00 AM" — one event, two spellings of the same instant.
-      hour12: true,
+      //
+      // `hourCycle: 'h12'` and NOT `hour12: true`, which is not the same request
+      // and does not survive a Node upgrade. For a locale whose default cycle is
+      // h23 — `en-GB` is one — Node 20 resolves `hour12: true` to **h11**, whose
+      // hours run 0–11, so midnight comes out "0:00 am"; Node 22 and later
+      // resolve it to h12 and it comes out "12:00 am". Same ICU on both, so this
+      // is V8's ECMA-402 behaviour and not locale data, and the image ships
+      // node:20-alpine 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. `recurrence.js` states the mirror-image rule for `h23`; there
+      // is no `hour12` left in this repo and it should stay that way.
+      hourCycle: 'h12',
     }).format(when)
     return `${text} (${zone || 'UTC'})`
   } catch {
diff --git a/server/test/eventAnnounce.test.js b/server/test/eventAnnounce.test.js
index bb72fc3..4d09e4c 100644
--- a/server/test/eventAnnounce.test.js
+++ b/server/test/eventAnnounce.test.js
@@ -260,9 +260,15 @@ test('the start time is written out in the SHARD\'s zone, not the server\'s', ()
 })
 
 test('midnight reads as 12:00 am and never as 00:00', () => {
-  // `hour12` is set explicitly. Left to the en-GB locale this would render
+  // The hour cycle is set explicitly. Left to the en-GB locale this would render
   // "00:00" while the schedule editor beside it writes "12:00 AM" — one event,
   // two spellings of the same instant.
+  //
+  // This assertion only has teeth on the Node the image ships (20), where
+  // `hour12: true` resolves to h11 and midnight reads "0:00 am". On Node 22+ it
+  // passes either way — so a green run on a dev machine is not evidence, and CI
+  // is what actually holds this line. See the note beside `hourCycle` in
+  // events/announce.js.
   assert.match(announce.startsAtLabel(new Date('2026-09-13T04:00:00Z'), 'America/New_York'), /12:00 am/)
 })