fix: the same document always saves to the same bytes

.changeset/deterministic-zip.md

@@ -0,0 +1,5 @@
+---
+"@office-kit/docx": patch
+---
+
+`toUint8Array` now produces the same bytes for the same document whenever it is called. The ZIP entries used to carry the time of saving, so two saves of an unchanged document could differ; every entry is now dated 1980-01-01 00:00 (the earliest ZIP date). Word and other readers ignore these dates.

src/api/idempotent-resave.test.ts

@@ -8,7 +8,7 @@
 // save+open cycles. If they differ, the library is non-deterministic
 // or re-flushes side parts unnecessarily, and we'd want to know.
 
-import { describe, expect, it } from "vitest";
+import { afterEach, describe, expect, it, vi } from "vitest";
 import {
   addBookmark,
   addBulletList,
@@ -98,4 +98,19 @@ describe("idempotent re-save", () => {
     const bytesC = reSaveCycle(bytesB);
     expect(bytesC).toEqual(bytesB);
   });
+
+  describe("saving at another time", () => {
+    afterEach(() => {
+      vi.useRealTimers();
+    });
+
+    it("produces the same bytes (no save time in the archive)", () => {
+      vi.useFakeTimers({ toFake: ["Date"] });
+      vi.setSystemTime(new Date(2026, 0, 1, 12, 0, 0));
+      const doc = createDocx({ paragraphs: ["body"] });
+      const first = toUint8Array(doc);
+      vi.setSystemTime(new Date(2026, 5, 30, 23, 59, 59));
+      expect(toUint8Array(doc)).toEqual(first);
+    });
+  });
 });

src/internal/opc/zip.ts

@@ -30,9 +30,22 @@ export interface WriteZipEntry {
  * fflate's terms); `"deflate"` uses level 6.
  */
 export function writeZip(entries: readonly WriteZipEntry[]): Uint8Array {
-  const input: Record<string, [Uint8Array, { level: 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 }]> = {};
+  const input: Record<string, [Uint8Array, ZipEntryOptions]> = {};
+  // Built per call: fflate encodes the local-time fields, and a Date made from
+  // them reads back the same in every time zone.
+  const mtime = new Date(ZIP_EPOCH_YEAR, 0, 1);
   for (const entry of entries) {
-    input[entry.name] = [entry.data, { level: entry.compression === "store" ? 0 : 6 }];
+    input[entry.name] = [entry.data, { level: entry.compression === "store" ? 0 : 6, mtime }];
   }
   return zipSync(input);
 }
+
+interface ZipEntryOptions {
+  level: 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9;
+  mtime: Date;
+}
+
+// Every entry carries this timestamp (the earliest a ZIP's DOS date can hold)
+// rather than the time of saving, which fflate uses by default: then the same
+// document always serializes to the same bytes. Word ignores entry times.
+const ZIP_EPOCH_YEAR = 1980;