diff --git a/src/components/settings/__tests__/environment-section-dates.test.tsx b/src/components/settings/__tests__/environment-section-dates.test.tsx
new file mode 100644
index 000000000..a37b6c670
--- /dev/null
+++ b/src/components/settings/__tests__/environment-section-dates.test.tsx
@@ -0,0 +1,58 @@
+/**
+ * The environment settings default their date fields to today. "Today" is
+ * the user's day: the UTC day was yesterday west of UTC in the evening and
+ * tomorrow east of it in the small hours.
+ */
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { renderToStaticMarkup } from "react-dom/server";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+
+import { I18nProvider } from "@/lib/i18n/context";
+
+const auth = vi.hoisted(() => ({ timezone: "America/Los_Angeles" }));
+vi.mock("@/hooks/use-auth", () => ({
+  useAuth: () => ({
+    user: { timezone: auth.timezone, modules: { environment: true } },
+    isAuthenticated: true,
+  }),
+}));
+vi.mock("next/navigation", () => ({
+  useRouter: () => ({ push: vi.fn(), replace: vi.fn() }),
+  usePathname: () => "/settings/environment",
+  useSearchParams: () => new URLSearchParams(""),
+}));
+
+import { EnvironmentSection } from "../environment-section";
+
+function backfillEnd(): string | undefined {
+  const client = new QueryClient({ defaultOptions: { queries: { retry: 0 } } });
+  const html = renderToStaticMarkup(
+    <QueryClientProvider client={client}>
+      <I18nProvider initialLocale="en">
+        <EnvironmentSection />
+      </I18nProvider>
+    </QueryClientProvider>,
+  );
+  return /id="env-backfill-end"[^>]*value="([^"]*)"/.exec(html)?.[1];
+}
+
+beforeEach(() => {
+  vi.useFakeTimers({ toFake: ["Date"] });
+});
+afterEach(() => {
+  vi.useRealTimers();
+});
+
+describe("<EnvironmentSection> date defaults", () => {
+  it("defaults to the user's day west of UTC", () => {
+    auth.timezone = "America/Los_Angeles";
+    vi.setSystemTime(new Date("2026-07-03T03:00:00.000Z")); // 2 July, 20:00
+    expect(backfillEnd()).toBe("2026-07-02");
+  });
+
+  it("defaults to the user's day east of UTC", () => {
+    auth.timezone = "Pacific/Kiritimati";
+    vi.setSystemTime(new Date("2026-07-02T12:00:00.000Z")); // 3 July, 02:00
+    expect(backfillEnd()).toBe("2026-07-03");
+  });
+});
diff --git a/src/components/settings/environment-section.tsx b/src/components/settings/environment-section.tsx
index 0a9de3eb6..166500a69 100644
--- a/src/components/settings/environment-section.tsx
+++ b/src/components/settings/environment-section.tsx
@@ -32,6 +32,12 @@ import { formatDateTime } from "@/lib/format";
 import { useTranslations } from "@/lib/i18n/context";
 import { apiDelete, apiGet, apiPost, apiPut } from "@/lib/api/api-fetch";
 import { queryKeys } from "@/lib/query-keys";
+import {
+  DEFAULT_TIMEZONE,
+  detectBrowserTimezone,
+  userDayKey,
+  validTimezoneOr,
+} from "@/lib/tz/format";
 
 interface GeocodeResult {
   lat: number;
@@ -72,14 +78,18 @@ interface EnvironmentOverview {
   attribution: string;
 }
 
-function todayKey(): string {
-  return new Date().toISOString().slice(0, 10);
-}
-
 export function EnvironmentSection() {
   const { t, tCount } = useTranslations();
   const { user } = useAuth();
   const queryClient = useQueryClient();
+  // Date defaults are the user's own days. The UTC day was tomorrow for
+  // anyone east of UTC in the small hours and yesterday west of it in the
+  // evening.
+  const tz = validTimezoneOr(
+    user?.timezone ?? detectBrowserTimezone(),
+    DEFAULT_TIMEZONE,
+  );
+  const todayKey = () => userDayKey(new Date(), tz);
 
   const enabled = user?.modules?.environment === true;
 
@@ -217,7 +227,7 @@ export function EnvironmentSection() {
   // user can still type an earlier start, but pre-home days resolve to SKIP on
   // the server — to fill the deep past they add an explicit location period.
   const effectiveBackfillStart =
-    backfillStart || home?.since?.slice(0, 10) || "";
+    backfillStart || (home?.since ? userDayKey(new Date(home.since), tz) : "");
 
   return (
     <div className="space-y-6">