diff --git a/.github/workflows/libraries.yml b/.github/workflows/libraries.yml
index 93050ab..1def6ae 100644
--- a/.github/workflows/libraries.yml
+++ b/.github/workflows/libraries.yml
@@ -1,7 +1,8 @@
 name: Library pack
 
 # Rebuilds packages/libraries from the H5P hub every week and opens a pull request when a library
-# changed. The pack is built deterministically, so an unchanged hub leaves the tree clean.
+# changed. The pack is built deterministically, in any time zone, so an unchanged hub leaves the
+# tree clean.
 
 on:
   schedule:
@@ -39,7 +40,17 @@ jobs:
           fi
 
           branch=libraries/refresh
-          changes=$(git diff -U0 -- packages/libraries/libraries.txt | grep '^[-+]H5P' | sort -k1.2 || true)
+          # Every library's line, `<folder>  <version>  <licence>`, FontAwesome and jQuery.ui among them.
+          changes=$(git diff -U0 -- packages/libraries/libraries.txt | grep -E '^[-+][A-Za-z][^ ]*-[0-9]+\.[0-9]+  ' | sort -k1.2 || true)
+          if [ -z "$changes" ]; then
+            # The files changed and no library's version, licence or authors did: the build is not
+            # deterministic, as it was once in the builder's time zone (#51), or the hub changed a
+            # library's files without a new version. Either way a pull request would say the hub
+            # changed when no library did.
+            echo "::error::The pack's files changed, but no library in libraries.txt did. Rebuild with pnpm libraries and compare the archives' entries before releasing anything."
+            git status --short -- packages/libraries | head -20
+            exit 1
+          fi
 
           version=$(node -e "
             const fs = require('fs'), path = 'packages/libraries/package.json'
diff --git a/AGENTS.md b/AGENTS.md
index 9d948bf..2cb13ce 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -1018,7 +1018,13 @@ element, and the element acts. That relay is why `frame-document.ts` has a `mess
   requires, its entries copied from the pack as they are (zip.js `passThrough`), so a part's bytes
   change only when its library does and the weekly refresh commits only those; `parts/index.json`
   says each library's full version, dependencies and addon patterns and its archive's size and
-  SHA-256. Measured on the demo's packages: 2 to 16 parts, 0.04 to 1.5 MB, 4.0 MB for formulas
+  SHA-256. Every entry of the pack and its parts carries `FIXED_DATE` (`scripts/parts.mjs`), built
+  from local fields, since a zip records a wall-clock time and zip.js reads the Date's local ones:
+  built from UTC fields, it wrote 02:00 where the pack was built at UTC+2 and 00:00 on CI, whose
+  first refresh after the split rewrote all 106 archives with no library changed (#48, closed
+  unmerged; #51). `tests/unit/pack-bytes.test.ts` in the player builds in four zones, and the
+  workflow stops with an error when the files change and no library's line in `libraries.txt`
+  does. Measured on the demo's packages: 2 to 16 parts, 0.04 to 1.5 MB, 4.0 MB for formulas
   (MathDisplay is 2.8 MB), against 14 MB whole. **Planning is the worker's** (`planFromPack` in
   `mount.ts`, the rules in `sw/pack-plan.ts`): an `index` that offers the pack (`pack` on the
   message: `{ pinned: true }` for `LIBRARY_PACK_BASE`, whose index the worker carries compiled in
diff --git a/apps/demo/scripts/build-libraries.mjs b/apps/demo/scripts/build-libraries.mjs
index 024422e..d066479 100644
--- a/apps/demo/scripts/build-libraries.mjs
+++ b/apps/demo/scripts/build-libraries.mjs
@@ -3,7 +3,7 @@ import { writeFile } from 'node:fs/promises'
 import { join, resolve } from 'node:path'
 import { BlobReader, BlobWriter, TextReader, Uint8ArrayReader, Uint8ArrayWriter, ZipReader, ZipWriter, configure } from '@zip.js/zip.js'
 import { formatBytes } from '@missing-elements/h5p-normalize'
-import { splitPack, writePackModule } from '../../../packages/libraries/scripts/parts.mjs'
+import { FIXED_DATE, splitPack, writePackModule } from '../../../packages/libraries/scripts/parts.mjs'
 import { HUB, download as cachedDownload, hubBundle as cachedBundle } from './lib/hub.mjs'
 
 /**
@@ -159,9 +159,10 @@ if (unknown.length > 0) {
 
 // A bundle registered to supply libraries is never held to its own manifest, but the player
 // refuses any archive without an h5p.json.
-// A fixed date, so the pack's bytes change only when a library does: the weekly refresh
-// (.github/workflows/libraries.yml) opens a pull request only on a real difference.
-const writer = new ZipWriter(new BlobWriter('application/zip'), { lastModDate: new Date(Date.UTC(2000, 0, 1)), extendedTimestamp: false })
+// A fixed date, the same in any time zone (see `FIXED_DATE`), so the pack's bytes change only when
+// a library does: the weekly refresh (.github/workflows/libraries.yml) opens a pull request only
+// on a real difference.
+const writer = new ZipWriter(new BlobWriter('application/zip'), { lastModDate: FIXED_DATE, extendedTimestamp: false })
 await writer.add(
   'h5p.json',
   new TextReader(
diff --git a/packages/libraries/scripts/parts.mjs b/packages/libraries/scripts/parts.mjs
index 66090b6..027d178 100644
--- a/packages/libraries/scripts/parts.mjs
+++ b/packages/libraries/scripts/parts.mjs
@@ -30,8 +30,15 @@ const packageDir = resolve(import.meta.dirname, '..')
  */
 const PART_MANIFEST = JSON.stringify({ title: 'H5P Offline Player library pack', language: 'und', mainLibrary: '', preloadedDependencies: [], embedTypes: ['div'] })
 
-/** A fixed date, as the pack has: the bytes change only when a library does. */
-const FIXED_DATE = new Date(Date.UTC(2000, 0, 1))
+/**
+ * The date every entry of the pack and its parts is written with, so the bytes change only when a
+ * library does. A zip records a wall-clock time with no zone, and zip.js takes it from the Date's
+ * local fields, so this is built from local fields too: the same bytes in any time zone. It was
+ * `Date.UTC(2000, 0, 1)` until #51, which wrote 02:00 at UTC+2, where the pack up to 1.0.0 was
+ * built, and 00:00 on CI, whose first weekly run rewrote all 106 archives with no library changed
+ * (#48). 02:00 is kept so the bytes stay those pack 1.0.0 shipped.
+ */
+export const FIXED_DATE = new Date(2000, 0, 1, 2)
 
 /**
  * The smallest a part is written. A bundler inlines a small asset named by `new URL` as a `data:`
diff --git a/packages/player/tests/unit/pack-bytes.test.ts b/packages/player/tests/unit/pack-bytes.test.ts
new file mode 100644
index 0000000..a3d8953
--- /dev/null
+++ b/packages/player/tests/unit/pack-bytes.test.ts
@@ -0,0 +1,71 @@
+import { execFileSync } from 'node:child_process'
+import { resolve } from 'node:path'
+import { pathToFileURL } from 'node:url'
+import { describe, expect, it } from 'vitest'
+
+/**
+ * The library pack's bytes, which the player pins by hash, depend on nothing but its libraries. A
+ * zip records a wall-clock time with no zone, and the pack was once written with a date built
+ * from UTC fields: the pack built at UTC+2 said 02:00 and CI's said 00:00, so the first weekly
+ * refresh after the split rewrote every archive with no library changed (#48, #51).
+ *
+ * Each time zone runs in a process of its own: whether setting `TZ` inside a running test worker
+ * moves its clock is the engine's business, and a child process starts in the zone it is given.
+ */
+
+const librariesDir = resolve(import.meta.dirname, '../../../libraries')
+const parts = pathToFileURL(resolve(librariesDir, 'scripts/parts.mjs')).href
+
+// A pack of one library, written the way apps/demo/scripts/build-libraries.mjs writes the real
+// one, split by the real `splitPack`; prints the bytes' hashes and the first entry's DOS time and date.
+const build = `
+import { createHash } from 'node:crypto'
+import { mkdtemp, readFile } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { BlobWriter, TextReader, ZipWriter, configure } from '@zip.js/zip.js'
+import { FIXED_DATE, splitPack } from ${JSON.stringify(parts)}
+
+configure({ useWebWorkers: false })
+const writer = new ZipWriter(new BlobWriter('application/zip'), { lastModDate: FIXED_DATE, extendedTimestamp: false })
+await writer.add('h5p.json', new TextReader('{}'))
+await writer.add('H5P.Tiny-1.0/library.json', new TextReader(JSON.stringify({ machineName: 'H5P.Tiny', majorVersion: 1, minorVersion: 0, patchVersion: 3 })))
+await writer.add('H5P.Tiny-1.0/tiny.js', new TextReader('H5P.Tiny = function () {}'))
+const pack = new Uint8Array(await (await writer.close()).arrayBuffer())
+
+const out = await mkdtemp(join(tmpdir(), 'pack-bytes-'))
+const [{ sha256 }] = await splitPack(pack, out)
+const part = await readFile(join(out, 'H5P.Tiny-1.0.h5p'))
+const header = new DataView(pack.buffer)
+console.log(JSON.stringify({
+  pack: createHash('sha256').update(pack).digest('hex'),
+  part: createHash('sha256').update(part).digest('hex'),
+  indexed: sha256,
+  time: header.getUint16(10, true),
+  date: header.getUint16(12, true)
+}))
+`
+
+function buildIn(timeZone: string) {
+  const output = execFileSync(process.execPath, ['--input-type=module', '-e', build], {
+    cwd: librariesDir,
+    env: { ...process.env, TZ: timeZone },
+    encoding: 'utf8'
+  })
+  return JSON.parse(output.trim().split('\n').at(-1)!) as { pack: string; part: string; indexed: string; time: number; date: number }
+}
+
+describe('the library pack', () => {
+  it('is the same bytes in every time zone', () => {
+    const [utc, ...others] = ['UTC', 'Europe/Kyiv', 'America/Los_Angeles', 'Pacific/Kiritimati'].map(buildIn)
+    for (const other of others) expect(other).toEqual(utc)
+    expect(utc.part).toBe(utc.indexed)
+  }, 30_000)
+
+  it('records 2000-01-01 02:00, the time pack 1.0.0 shipped with', () => {
+    // A different time rewrites every part and every hash the player pins, for no library's sake.
+    const { time, date } = buildIn('UTC')
+    expect(time).toBe(2 << 11)
+    expect(date).toBe(((2000 - 1980) << 9) | (1 << 5) | 1)
+  }, 30_000)
+})