diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index eb8909465..2533625aa 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -47119,6 +47119,9 @@ components:
             - type: "null"
         timezone:
           type: string
+          description: "The IANA zone the server cuts this account's days in, the same value `/api/auth/me` reports: the stored
+            zone, or the instance default when the stored value is not a usable IANA name (since v1.39.1; before, the
+            raw stored value). Always a valid IANA name, so sending it back in a PATCH is accepted."
         timeFormat:
           type: string
           enum:
@@ -47231,6 +47234,9 @@ components:
             - type: "null"
         timezone:
           type: string
+          description: "The IANA zone the server cuts this account's days in, the same value `/api/auth/me` reports: the stored
+            zone, or the instance default when the stored value is not a usable IANA name (since v1.39.1; before, the
+            raw stored value). Always a valid IANA name, so sending it back in a PATCH is accepted."
         timeFormat:
           type: string
           enum:
diff --git a/src/app/api/auth/me/route.ts b/src/app/api/auth/me/route.ts
index 0c7ac8244..2dfa2bc58 100644
--- a/src/app/api/auth/me/route.ts
+++ b/src/app/api/auth/me/route.ts
@@ -49,10 +49,7 @@ import { syncMfaEnrollCookie } from "@/lib/auth/mfa-enrollment";
 import { buildAvatarUrl } from "@/lib/avatar";
 import { decrypt } from "@/lib/crypto";
 import { prisma } from "@/lib/db";
-import {
-  isValidTimezone,
-  resolveServerDefaultTimezone,
-} from "@/lib/tz/resolver";
+import { resolveStoredTimezone } from "@/lib/tz/resolver";
 import { isCycleEnabled } from "@/lib/cycle/gate";
 import {
   resolveModuleMap,
@@ -279,9 +276,7 @@ export const GET = apiHandler(async () => {
     // The zone the server cuts this account's days in: the stored zone, or
     // the instance default when the stored one is unusable. Clients bucket
     // by this value, so it must be the resolved one, not the raw column.
-    timezone: isValidTimezone(user.timezone)
-      ? user.timezone
-      : await resolveServerDefaultTimezone(),
+    timezone: await resolveStoredTimezone(user.timezone),
     onboardingCompletedAt: user.onboardingCompletedAt,
     onboardingTourCompleted: user.onboardingTourCompleted,
     // v1.39 (C1) — the needs-based setup flow, published additively:
diff --git a/src/app/api/user/profile/__tests__/route.test.ts b/src/app/api/user/profile/__tests__/route.test.ts
index f771581db..03567bd91 100644
--- a/src/app/api/user/profile/__tests__/route.test.ts
+++ b/src/app/api/user/profile/__tests__/route.test.ts
@@ -4,6 +4,7 @@ import { NextRequest } from "next/server";
 vi.mock("@/lib/db", () => ({
   prisma: {
     user: { findUnique: vi.fn(), update: vi.fn() },
+    appSettings: { findUnique: vi.fn() },
   },
 }));
 
@@ -60,6 +61,12 @@ const SESSION_OK = {
 beforeEach(() => {
   vi.resetAllMocks();
   vi.mocked(resolveModuleMap).mockResolvedValue(MODULE_MAP as never);
+  // The operator's default zone, which the server falls back to when a
+  // stored one is unusable. Distinct from the built-in default so a test can
+  // tell the resolver ran.
+  vi.mocked(prisma.appSettings.findUnique).mockResolvedValue({
+    defaultUserTimezone: "America/Chicago",
+  } as never);
 });
 
 const callGet = GET as unknown as (req: NextRequest) => Promise<Response>;
@@ -438,3 +445,81 @@ describe("PATCH /api/user/profile", () => {
     ]);
   });
 });
+
+/**
+ * `/api/auth/me` has returned the zone the server actually cuts days in —
+ * the stored one, or the instance default when the stored value is unusable
+ * — while this alias returned the raw column. A client reading this endpoint
+ * could bucket a day differently from the server. Both now go through the
+ * same resolution.
+ */
+describe("/api/user/profile — timezone is the resolved zone", () => {
+  function profileRow(timezone: string | null) {
+    return {
+      username: "testuser",
+      displayName: null,
+      email: null,
+      dateOfBirth: null,
+      gender: null,
+      heightCm: null,
+      locale: null,
+      timezone,
+      moodReminderEnabled: false,
+      fullName: null,
+      insurerName: null,
+      insurerIkNumber: null,
+      insuranceNumberEncrypted: null,
+    };
+  }
+
+  async function getZone(stored: string | null): Promise<string> {
+    vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+    vi.mocked(prisma.user.findUnique).mockResolvedValue(
+      profileRow(stored) as never,
+    );
+    const res = await callGet(makeGetReq());
+    expect(res.status).toBe(200);
+    return ((await res.json()) as { data: { timezone: string } }).data.timezone;
+  }
+
+  it("returns a usable stored zone as stored", async () => {
+    expect(await getZone("Asia/Tokyo")).toBe("Asia/Tokyo");
+  });
+
+  it("returns the instance default in place of a bare offset", async () => {
+    expect(await getZone("+05:30")).toBe("America/Chicago");
+  });
+
+  it("returns the instance default in place of a name no runtime knows", async () => {
+    expect(await getZone("Mars/Olympus_Mons")).toBe("America/Chicago");
+  });
+
+  it("returns the instance default when the row is missing", async () => {
+    vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+    vi.mocked(prisma.user.findUnique).mockResolvedValue(null);
+    const res = await callGet(makeGetReq());
+    const body = (await res.json()) as { data: { timezone: string } };
+    expect(body.data.timezone).toBe("America/Chicago");
+  });
+
+  it("answers a PATCH that leaves an unusable zone untouched with the resolved one", async () => {
+    vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+    vi.mocked(prisma.user.findUnique).mockResolvedValue(null);
+    vi.mocked(prisma.user.update).mockResolvedValue({
+      ...profileRow("+05:30"),
+      id: "user-1",
+      role: "USER",
+      displayName: "Alex T.",
+    } as never);
+    const res = await PATCH(
+      new NextRequest("http://localhost/api/user/profile", {
+        method: "PATCH",
+        headers: { "content-type": "application/json" },
+        body: JSON.stringify({ displayName: "Alex T." }),
+      }),
+    );
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as { data: { timezone: string } };
+    expect(body.data.timezone).toBe("America/Chicago");
+  });
+});
diff --git a/src/app/api/user/profile/route.ts b/src/app/api/user/profile/route.ts
index d6e6f6a71..38e3dd77f 100644
--- a/src/app/api/user/profile/route.ts
+++ b/src/app/api/user/profile/route.ts
@@ -20,6 +20,7 @@ import { applyProfileUpdate } from "@/lib/auth/profile-update";
 import { prisma } from "@/lib/db";
 import { decrypt } from "@/lib/crypto";
 import { resolveModuleMap } from "@/lib/modules/gate";
+import { resolveStoredTimezone } from "@/lib/tz/resolver";
 
 export const dynamic = "force-dynamic";
 
@@ -77,7 +78,10 @@ export const GET = apiHandler(async () => {
     gender: dbUser?.gender ?? null,
     heightCm: dbUser?.heightCm ?? null,
     locale: dbUser?.locale ?? null,
-    timezone: dbUser?.timezone ?? "Europe/Berlin",
+    // The zone the server cuts this account's days in, exactly as
+    // `/api/auth/me` reports it: the stored zone, or the instance default
+    // when the stored value is unusable. Never the raw column.
+    timezone: await resolveStoredTimezone(dbUser?.timezone),
     timeFormat: dbUser?.timeFormat ?? "AUTO",
     dateFormat: dbUser?.dateFormat ?? "AUTO",
     moodReminderEnabled: dbUser?.moodReminderEnabled ?? false,
@@ -118,7 +122,7 @@ export const PATCH = apiHandler(async (request: NextRequest) => {
     gender: result.user.gender,
     heightCm: result.user.heightCm,
     locale: result.user.locale,
-    timezone: result.user.timezone,
+    timezone: await resolveStoredTimezone(result.user.timezone),
     timeFormat: result.user.timeFormat,
     dateFormat: result.user.dateFormat,
     moodReminderEnabled: result.user.moodReminderEnabled,
diff --git a/src/lib/openapi/routes/profile.ts b/src/lib/openapi/routes/profile.ts
index f6b1dba67..b1fa0100d 100644
--- a/src/lib/openapi/routes/profile.ts
+++ b/src/lib/openapi/routes/profile.ts
@@ -498,7 +498,11 @@ const profileResponse = z
     gender: z.enum(["MALE", "FEMALE", "OTHER"]).nullable(),
     heightCm: z.number().nullable(),
     locale: z.string().nullable(),
-    timezone: z.string(),
+    timezone: z
+      .string()
+      .describe(
+        "The IANA zone the server cuts this account's days in, the same value `/api/auth/me` reports: the stored zone, or the instance default when the stored value is not a usable IANA name (since v1.39.1; before, the raw stored value). Always a valid IANA name, so sending it back in a PATCH is accepted.",
+      ),
     timeFormat: z
       .enum(["AUTO", "H12", "H24"])
       .describe(
@@ -542,7 +546,11 @@ const profileUpdateResponse = z
     gender: z.enum(["MALE", "FEMALE", "OTHER"]).nullable(),
     heightCm: z.number().nullable(),
     locale: z.string().nullable(),
-    timezone: z.string(),
+    timezone: z
+      .string()
+      .describe(
+        "The IANA zone the server cuts this account's days in, the same value `/api/auth/me` reports: the stored zone, or the instance default when the stored value is not a usable IANA name (since v1.39.1; before, the raw stored value). Always a valid IANA name, so sending it back in a PATCH is accepted.",
+      ),
     timeFormat: z
       .enum(["AUTO", "H12", "H24"])
       .describe(
diff --git a/src/lib/tz/resolver.ts b/src/lib/tz/resolver.ts
index 1835432d5..6f3034dc5 100644
--- a/src/lib/tz/resolver.ts
+++ b/src/lib/tz/resolver.ts
@@ -144,6 +144,21 @@ export async function resolveServerDefaultTimezone(): Promise<string> {
   return tz;
 }
 
+/**
+ * The zone the server actually uses for a stored `User.timezone` value: the
+ * value itself when it is a usable IANA name, the instance default otherwise.
+ *
+ * Every response that reports an account's zone goes through this, so a
+ * client never buckets days by a value the server itself has set aside.
+ */
+export async function resolveStoredTimezone(
+  stored: string | null | undefined,
+): Promise<string> {
+  return stored && isValidTimezone(stored)
+    ? stored
+    : resolveServerDefaultTimezone();
+}
+
 /**
  * Resolve the per-user display timezone. Reads `User.timezone`. Falls
  * back to the server default and then to "Europe/Berlin". Cached for