diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index 5953a81d8..a739c5947 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -27454,9 +27454,13 @@ components:
             - type: "null"
         anchorDate:
           anyOf:
-            - 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)))$
+            - anyOf:
+                - type: string
+                  format: date
+                  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])))$
+                - 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)))$
             - type: "null"
         notifyHour:
           default: 9
@@ -27528,9 +27532,13 @@ components:
             - type: "null"
         anchorDate:
           anyOf:
-            - 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)))$
+            - anyOf:
+                - type: string
+                  format: date
+                  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])))$
+                - 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)))$
             - type: "null"
         notifyHour:
           type: integer
diff --git a/src/app/api/measurement-reminders/[id]/route.ts b/src/app/api/measurement-reminders/[id]/route.ts
index 39228567a..9922ff88c 100644
--- a/src/app/api/measurement-reminders/[id]/route.ts
+++ b/src/app/api/measurement-reminders/[id]/route.ts
@@ -29,6 +29,7 @@ import { annotate } from "@/lib/logging/context";
 import { updateMeasurementReminderSchema } from "@/lib/validations/measurement-reminders";
 import {
   computeReminderNextDueAt,
+  parseReminderAnchor,
   type ReminderScheduleInput,
 } from "@/lib/measurement-reminders/scheduling";
 import { toMeasurementReminderDto } from "@/lib/measurement-reminders/dto";
@@ -141,9 +142,12 @@ export const PATCH = apiHandler(
         updateData.intervalDays = null;
       }
     }
+    const timezone = await resolveTimezone(user.id);
     if (data.anchorDate !== undefined) {
       updateData.anchorDate =
-        data.anchorDate != null ? new Date(data.anchorDate) : null;
+        data.anchorDate != null
+          ? parseReminderAnchor(data.anchorDate, timezone)
+          : null;
     }
     if (data.notifyHour !== undefined) updateData.notifyHour = data.notifyHour;
     if (data.location !== undefined) updateData.location = data.location;
@@ -166,7 +170,6 @@ export const PATCH = apiHandler(
     // auto-clear a few lines up — so keying off its own keys (present, even
     // when the value is `null`) can never diverge from what the database
     // ends up holding.
-    const timezone = await resolveTimezone(user.id);
     const now = new Date();
     const merged: ReminderScheduleInput = {
       intervalDays: Object.hasOwn(updateData, "intervalDays")
@@ -193,10 +196,10 @@ export const PATCH = apiHandler(
     // open, overdue check-up quietly moved it to its next slot. Compared by
     // value, because a client may resend every field on each save.
     //
-    // The first due date is compared by CALENDAR DAY in the profile zone. The
-    // web form sends a date as the browser's local midnight, and a booster
-    // row stores the dose instant plus N days, so the same day arrives as a
-    // different instant. A resent same-day anchor is dropped from the write
+    // The first due date is compared by CALENDAR DAY in the profile zone. A
+    // client may send a date-time (older web builds sent the browser's local
+    // midnight), and a booster row stores the dose instant plus N days, so the
+    // same day arrives as a different instant. A resent same-day anchor is dropped from the write
     // altogether, so the stored instant stays what it was.
     const sameInstant = (a: Date | null, b: Date | null) =>
       (a?.getTime() ?? null) === (b?.getTime() ?? null);
diff --git a/src/app/api/measurement-reminders/route.ts b/src/app/api/measurement-reminders/route.ts
index 06fe23443..150bbe793 100644
--- a/src/app/api/measurement-reminders/route.ts
+++ b/src/app/api/measurement-reminders/route.ts
@@ -21,6 +21,7 @@ import { withIdempotency } from "@/lib/idempotency";
 import { createMeasurementReminderSchema } from "@/lib/validations/measurement-reminders";
 import {
   computeReminderNextDueAt,
+  parseReminderAnchor,
   type ReminderScheduleInput,
 } from "@/lib/measurement-reminders/scheduling";
 import { toMeasurementReminderDto } from "@/lib/measurement-reminders/dto";
@@ -88,7 +89,10 @@ async function postReminder(request: NextRequest): Promise<Response> {
 
   const timezone = await resolveTimezone(user.id);
   const now = new Date();
-  const anchorDate = data.anchorDate != null ? new Date(data.anchorDate) : null;
+  const anchorDate =
+    data.anchorDate != null
+      ? parseReminderAnchor(data.anchorDate, timezone)
+      : null;
 
   // Server-authoritative next-due. No satisfy yet → anchors on
   // anchorDate ?? createdAt (createdAt ≈ now for a fresh row).
diff --git a/src/components/measurement-reminders/vorsorge-section.tsx b/src/components/measurement-reminders/vorsorge-section.tsx
index 1674b2457..f1085e2c5 100644
--- a/src/components/measurement-reminders/vorsorge-section.tsx
+++ b/src/components/measurement-reminders/vorsorge-section.tsx
@@ -181,11 +181,13 @@ function toDateInputValue(iso: string | null): string {
   return new Date(d.getTime() - offset * 60 * 1000).toISOString().slice(0, 10);
 }
 
-/** `<input type="date">` value → an ISO instant at local midnight, or null. */
+/**
+ * `<input type="date">` value → the calendar date itself, or null. The server
+ * reads a bare date as that day in the profile's zone, so a browser set to
+ * another zone cannot move the first-due day.
+ */
 function fromDateInputValue(value: string): string | null {
-  if (!value) return null;
-  const d = new Date(`${value}T00:00:00`);
-  return Number.isNaN(d.getTime()) ? null : d.toISOString();
+  return /^\d{4}-\d{2}-\d{2}$/.test(value) ? value : null;
 }
 
 interface FormState {
diff --git a/src/lib/measurement-reminders/__tests__/scheduling-timezones.test.ts b/src/lib/measurement-reminders/__tests__/scheduling-timezones.test.ts
index 560d08bbc..7cd923cc4 100644
--- a/src/lib/measurement-reminders/__tests__/scheduling-timezones.test.ts
+++ b/src/lib/measurement-reminders/__tests__/scheduling-timezones.test.ts
@@ -9,7 +9,7 @@ import { describe, expect, it } from "vitest";
 
 import { wallClockInTz, zonedWallClockToUtc } from "@/lib/tz/wall-clock";
 
-import { computeReminderNextDueAt } from "../scheduling";
+import { computeReminderNextDueAt, parseReminderAnchor } from "../scheduling";
 
 const ZONES = [
   "America/New_York",
@@ -75,3 +75,29 @@ describe.each(ZONES)("reminder anchors in %s", (tz) => {
     expect(localLabel(next, tz)).toBe("2026-09-20 19:00 w0");
   });
 });
+
+describe.each(ZONES)("a first-due date sent as a calendar date in %s", (tz) => {
+  it("is that day in the user's zone", () => {
+    const anchor = parseReminderAnchor("2026-10-01", tz);
+    expect(anchor).toEqual(localMidnight(tz, 2026, 10, 1));
+    const next = computeReminderNextDueAt(
+      {
+        intervalDays: 30,
+        rrule: null,
+        anchorDate: anchor,
+        notifyHour: 9,
+        lastSatisfiedAt: null,
+        createdAt: new Date("2026-09-01T12:00:00Z"),
+      },
+      tz,
+      new Date("2026-09-26T12:00:00Z"),
+    );
+    expect(localLabel(next, tz)?.slice(0, 16)).toBe("2026-10-01 09:00");
+  });
+
+  it("keeps a date-time as the instant it names", () => {
+    expect(parseReminderAnchor("2026-10-01T00:00:00.000Z", tz)).toEqual(
+      new Date("2026-10-01T00:00:00.000Z"),
+    );
+  });
+});
diff --git a/src/lib/measurement-reminders/scheduling.ts b/src/lib/measurement-reminders/scheduling.ts
index 2227d0008..49cbf8aaf 100644
--- a/src/lib/measurement-reminders/scheduling.ts
+++ b/src/lib/measurement-reminders/scheduling.ts
@@ -31,6 +31,7 @@ import {
 } from "@/lib/medications/scheduling/recurrence";
 import { wallClockInTz } from "@/lib/tz/wall-clock";
 import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
+import { startOfLocalDayKey } from "@/lib/tz/local-day";
 
 /**
  * The reminder fields this module reads. A subset of the Prisma row so
@@ -52,6 +53,18 @@ export interface ReminderScheduleInput {
   endsOn?: Date | null;
 }
 
+/**
+ * A first-due date from a request: a calendar date (`YYYY-MM-DD`) is that
+ * day in the user's zone, stored as its local midnight; a date-time is the
+ * instant it names. A date sent as UTC midnight would read as the evening
+ * before west of UTC, which is why a bare date is accepted at all.
+ */
+export function parseReminderAnchor(value: string, timeZone: string): Date {
+  return /^\d{4}-\d{2}-\d{2}$/.test(value)
+    ? startOfLocalDayKey(value, timeZone || DEFAULT_TIMEZONE)
+    : new Date(value);
+}
+
 /** The calendar day `instant` falls on in `tz`, as UTC midnight of that date. */
 function calendarDayInZone(instant: Date, tz: string): Date {
   const p = wallClockInTz(instant, tz);
diff --git a/src/lib/validations/__tests__/measurement-reminders.test.ts b/src/lib/validations/__tests__/measurement-reminders.test.ts
index aa013d421..78881ba55 100644
--- a/src/lib/validations/__tests__/measurement-reminders.test.ts
+++ b/src/lib/validations/__tests__/measurement-reminders.test.ts
@@ -118,3 +118,19 @@ describe("measurementReminderTypeEnum (V3 — completeness)", () => {
     expect(parsed.success).toBe(true);
   });
 });
+
+describe("createMeasurementReminderSchema — first-due date", () => {
+  const base = { label: "Blood test", intervalDays: 365 };
+  it.each([
+    ["2026-10-01", true],
+    ["2026-10-01T00:00:00.000Z", true],
+    ["2026-10-01T00:00:00-07:00", true],
+    ["2026-10-01T00:00:00", false],
+    ["01.10.2026", false],
+  ])("anchorDate %s is accepted: %s", (anchorDate, ok) => {
+    expect(
+      createMeasurementReminderSchema.safeParse({ ...base, anchorDate })
+        .success,
+    ).toBe(ok);
+  });
+});
diff --git a/src/lib/validations/measurement-reminders.ts b/src/lib/validations/measurement-reminders.ts
index 724e2b2d4..a329afa46 100644
--- a/src/lib/validations/measurement-reminders.ts
+++ b/src/lib/validations/measurement-reminders.ts
@@ -143,13 +143,22 @@ const cadenceRefinement = (
   }
 };
 
+/**
+ * The first-due date: a calendar date (`YYYY-MM-DD`, read as that day in the
+ * user's zone) or a date-time (the instant it names).
+ */
+const anchorDateField = z.union([
+  z.iso.date(),
+  z.iso.datetime({ offset: true }),
+]);
+
 export const createMeasurementReminderSchema = z
   .object({
     label: z.string().trim().min(1).max(120),
     measurementType: measurementReminderTypeEnum.nullish(),
     intervalDays: z.number().int().min(1).max(3650).nullish(),
     rrule: rruleField.nullish(),
-    anchorDate: z.iso.datetime({ offset: true }).nullish(),
+    anchorDate: anchorDateField.nullish(),
     notifyHour: z.number().int().min(0).max(23).default(9),
     location: z.string().trim().max(200).nullish(),
     enabled: z.boolean().default(true),
@@ -177,7 +186,7 @@ export const updateMeasurementReminderSchema = z
     measurementType: measurementReminderTypeEnum.nullish(),
     intervalDays: z.number().int().min(1).max(3650).nullish(),
     rrule: rruleField.nullish(),
-    anchorDate: z.iso.datetime({ offset: true }).nullish(),
+    anchorDate: anchorDateField.nullish(),
     notifyHour: z.number().int().min(0).max(23).optional(),
     location: z.string().trim().max(200).nullish(),
     enabled: z.boolean().optional(),