diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index 3dcd229cc..0af40fc6b 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -19968,6 +19968,8 @@ components:
           type: string
           format: date-time
           pattern: ^(?:(?:\d\d[2468][048]|\d\d[13579][26]|\d\d0[48]|[02468][048]00|[13579][26]00)-02-29|\d{4}-(?:(?:0[13578]|1[02])-(?:0[1-9]|[12]\d|3[01])|(?:0[469]|11)-(?:0[1-9]|[12]\d|30)|(?:02)-(?:0[1-9]|1\d|2[0-8])))T(?:(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d(?:\.\d+)?)?(?:Z|([+-](?:[01]\d|2[0-3]):[0-5]\d)))$
+        dayKey:
+          type: string
         durationSec:
           type: integer
           minimum: 0
@@ -20076,6 +20078,7 @@ components:
         - sportType
         - startedAt
         - endedAt
+        - dayKey
         - durationSec
         - distanceM
         - activeEnergyKcal
diff --git a/src/app/api/workouts/[id]/route.ts b/src/app/api/workouts/[id]/route.ts
index 439dbe5f2..beaeeba12 100644
--- a/src/app/api/workouts/[id]/route.ts
+++ b/src/app/api/workouts/[id]/route.ts
@@ -41,6 +41,8 @@ import {
 import { computeSplits } from "@/lib/workouts/splits";
 import { buildSportContext } from "@/lib/workouts/sport-context";
 import type { RouteCoordinate } from "@/lib/workouts/route-svg";
+import { resolveUserTimezone } from "@/lib/tz/resolver";
+import { userDayKey } from "@/lib/tz/format";
 
 type RouteParams = { params: Promise<{ id: string }> };
 
@@ -129,6 +131,14 @@ export const GET = apiHandler(
       where: { id: user.id },
       select: { sourcePriorityJson: true, dateOfBirth: true },
     });
+    // The local calendar day this session belongs to. Resolved here, in the
+    // user's timezone, so the day linkage stays server-authoritative — a
+    // client that derived it from `startedAt` would fall back to the browser
+    // zone and put a late-evening session on the wrong day whenever the two
+    // disagree.
+    const timezone = await resolveUserTimezone(user.id);
+    const dayKey = userDayKey(row.startedAt, timezone);
+
     const canonicalCluster = pickCanonicalWorkoutRows(
       clusterRows,
       userRow?.sourcePriorityJson ?? null,
@@ -250,6 +260,10 @@ export const GET = apiHandler(
       sportType: row.sportType,
       startedAt: row.startedAt,
       endedAt: row.endedAt,
+      // `YYYY-MM-DD` in the user's timezone. Additive; it addresses the
+      // day-scoped reads (intraday pulse, that night's sleep, the day's
+      // mood) the detail surface renders around the session.
+      dayKey,
       durationSec: row.durationSec,
       distanceM: row.totalDistanceM,
       activeEnergyKcal: row.totalEnergyKcal,
diff --git a/src/app/api/workouts/__tests__/detail.test.ts b/src/app/api/workouts/__tests__/detail.test.ts
index 28981227b..830812867 100644
--- a/src/app/api/workouts/__tests__/detail.test.ts
+++ b/src/app/api/workouts/__tests__/detail.test.ts
@@ -15,7 +15,10 @@
  *   - the optional `route` field flows through GeoJSON geometry when a
  *     `WorkoutRoute` row is attached, and is null when missing;
  *   - `aiInsight` is a pure READ: it serves a stored paragraph when one
- *     exists, null when none does, and never generates one.
+ *     exists, null when none does, and never generates one;
+ *   - `dayKey` dates the session in the user's timezone, so a session that
+ *     starts late in the UTC evening lands on the local day the person
+ *     actually trained on.
  */
 import { afterEach, describe, expect, it, vi, beforeEach } from "vitest";
 import { NextRequest } from "next/server";
@@ -65,6 +68,7 @@ import { GET } from "../[id]/route";
 import { prisma } from "@/lib/db";
 import { getSession } from "@/lib/auth/session";
 import { isModuleEnabled, requireModuleEnabled } from "@/lib/modules/gate";
+import { invalidateUserTimezone } from "@/lib/tz/resolver";
 
 const SESSION_OK = {
   session: { id: "sess-1", expiresAt: new Date(Date.now() + 3_600_000) },
@@ -125,6 +129,9 @@ describe("GET /api/workouts/{id}", () => {
       enabled: true,
     } as never);
     vi.mocked(isModuleEnabled).mockResolvedValue(true);
+    // The timezone resolver caches per user for 60 s in process; drop the
+    // entry so a test that pins a zone is not served the previous one.
+    invalidateUserTimezone(SESSION_OK.user.id);
   });
 
   it("returns the workout with iOS-contract field names", async () => {
@@ -153,6 +160,36 @@ describe("GET /api/workouts/{id}", () => {
     expect(body.data.route).toBeNull();
   });
 
+  it("dates the session by the user's timezone, not by UTC", async () => {
+    // 20:00 UTC is already the NEXT calendar day in Auckland. A client that
+    // sliced the ISO timestamp would read 2026-05-15 and then pull the wrong
+    // day's pulse, sleep and mood around the session.
+    vi.mocked(prisma.user.findUnique).mockResolvedValue({
+      sourcePriorityJson: null,
+      dateOfBirth: null,
+      timezone: "Pacific/Auckland",
+    } as never);
+    vi.mocked(prisma.workout.findUnique).mockResolvedValueOnce({
+      ...BASE_ROW,
+      startedAt: new Date("2026-05-15T20:00:00Z"),
+      endedAt: new Date("2026-05-15T20:30:00Z"),
+    } as never);
+    vi.mocked(prisma.workout.findMany).mockResolvedValueOnce([
+      {
+        id: "w-1",
+        source: "APPLE_HEALTH",
+        startedAt: new Date("2026-05-15T20:00:00Z"),
+        sportType: "RUNNING",
+      },
+    ] as never);
+
+    const res = await GET(makeRequest(), makeParams("w-1"));
+    const body = await res.json();
+
+    expect(res.status).toBe(200);
+    expect(body.data.dayKey).toBe("2026-05-16");
+  });
+
   it("enriches the Apple winner with only matched WHOOP HR and zones", async () => {
     const routeGeometry = {
       type: "LineString",
diff --git a/src/hooks/use-workouts.ts b/src/hooks/use-workouts.ts
index 91cf885fd..1295f33cd 100644
--- a/src/hooks/use-workouts.ts
+++ b/src/hooks/use-workouts.ts
@@ -290,6 +290,14 @@ export type WorkoutActivityInsight = {
 } | null;
 
 export interface WorkoutDetailPayload extends WorkoutListEntry {
+  /**
+   * The session's local calendar day, `YYYY-MM-DD`. Computed server-side
+   * from `startedAt` in the user's timezone — never re-derived here, so a
+   * browser in a different zone can't move the workout to another day. It
+   * addresses the day-scoped reads the detail page renders around the
+   * session.
+   */
+  dayKey: string;
   minHr: number | null;
   stepCount: number | null;
   elevationM: number | null;
diff --git a/src/lib/openapi/routes/workouts.ts b/src/lib/openapi/routes/workouts.ts
index 51fdc4c45..d23980ff2 100644
--- a/src/lib/openapi/routes/workouts.ts
+++ b/src/lib/openapi/routes/workouts.ts
@@ -166,6 +166,13 @@ const workoutDetailResponse = z
     sportType: z.string(),
     startedAt: z.iso.datetime({ offset: true }),
     endedAt: z.iso.datetime({ offset: true }),
+    /**
+     * The local calendar day the session belongs to, `YYYY-MM-DD`, resolved
+     * server-side from `startedAt` in the user's timezone. Clients address
+     * the day-scoped reads with it instead of deriving a day from the
+     * timestamp in their own zone.
+     */
+    dayKey: z.string(),
     durationSec: z.number().int().nonnegative(),
     distanceM: z.number().nullable(),
     activeEnergyKcal: z.number().nullable(),