diff --git a/src/library/read.ts b/src/library/read.ts
index 3af8069..3c756b4 100644
--- a/src/library/read.ts
+++ b/src/library/read.ts
@@ -151,8 +151,8 @@ export interface PhotoFilter {
 }
 
 export interface TimelineGroup {
-    // The period's identity: `YYYY-MM-DD` for day and week (the Monday), and
-    // `YYYY-MM` for month.
+    // The period's identity: `YYYY-MM-DD` for day, `YYYY-Www` (ISO 8601 week,
+    // e.g. `2025-W32`) for week, and `YYYY-MM` for month.
     key: string;
     // Local-time milliseconds at the start of the period.
     startsAt: number;
@@ -292,6 +292,31 @@ const pad = (n: number): string => String(n).padStart(2, "0");
 const dateKey = (d: Date): string =>
     `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}`;
 
+const WEEK_MS = 7 * 24 * 60 * 60 * 1000;
+
+// The ISO 8601 week key `YYYY-Www` for the week starting at the given Monday.
+// The week-year is the year of that week's Thursday, so it can differ from the
+// calendar year at the January/December boundary (e.g. 2024-12-30 is 2025-W01).
+const isoWeekKey = (monday: Date): string => {
+    const thursday = new Date(
+        monday.getFullYear(),
+        monday.getMonth(),
+        monday.getDate() + 3,
+    );
+    const isoYear = thursday.getFullYear();
+    // Thursday of ISO week 1 is the Thursday of the week containing January 4.
+    const jan4 = new Date(isoYear, 0, 4);
+    const week1Thursday = new Date(
+        isoYear,
+        0,
+        4 + 3 - ((jan4.getDay() + 6) % 7),
+    );
+    const week =
+        1 +
+        Math.round((thursday.getTime() - week1Thursday.getTime()) / WEEK_MS);
+    return `${isoYear}-W${pad(week)}`;
+};
+
 // The period a millisecond instant falls in, in local time. Weeks start on
 // Monday. `Date` normalizes out-of-range day arguments, so the week's Monday
 // is correct across month and year boundaries.
@@ -312,7 +337,7 @@ const periodOf = (
         // getDay(): 0=Sunday..6=Saturday; shift so Monday is the week start.
         const fromMonday = (d.getDay() + 6) % 7;
         const start = new Date(year, month, day - fromMonday);
-        return { key: dateKey(start), startsAt: start.getTime() };
+        return { key: isoWeekKey(start), startsAt: start.getTime() };
     }
     const start = new Date(year, month, day);
     return { key: dateKey(start), startsAt: start.getTime() };
diff --git a/test/library/read.test.ts b/test/library/read.test.ts
index d8ad24f..102bdee 100644
--- a/test/library/read.test.ts
+++ b/test/library/read.test.ts
@@ -287,8 +287,9 @@ describe("lib.timeline grouping", () => {
             ],
         );
         const groups = apis(records).timeline.groups({ groupBy: "week" });
-        expect(groups.map((g) => g.key)).toEqual(["2024-01-22", "2024-01-15"]);
-        const first = groups.find((g) => g.key === "2024-01-15")!;
+        // ISO week keys: 2024-01-15 is in 2024-W03, the next Monday in 2024-W04.
+        expect(groups.map((g) => g.key)).toEqual(["2024-W04", "2024-W03"]);
+        const first = groups.find((g) => g.key === "2024-W03")!;
         expect(first.startsAt).toBe(Date.UTC(2024, 0, 15));
         // The Sunday belongs to the Monday-started week, not the next one.
         expect(first.fileIDs.sort((a, b) => a - b)).toEqual([1, 2, 3]);
@@ -301,7 +302,7 @@ describe("lib.timeline grouping", () => {
             [file(1, 1, { creationTime: micros(Date.UTC(2024, 0, 14, 12)) })],
         );
         const groups = apis(records).timeline.groups({ groupBy: "week" });
-        expect(groups.map((g) => g.key)).toEqual(["2024-01-08"]);
+        expect(groups.map((g) => g.key)).toEqual(["2024-W02"]);
         expect(groups[0]!.startsAt).toBe(Date.UTC(2024, 0, 8));
     });