diff --git a/src/lib/vaccinations/__tests__/satisfy-matching.test.ts b/src/lib/vaccinations/__tests__/satisfy-matching.test.ts
new file mode 100644
index 000000000..6c241c304
--- /dev/null
+++ b/src/lib/vaccinations/__tests__/satisfy-matching.test.ts
@@ -0,0 +1,62 @@
+/**
+ * Logging the next dose re-anchors a booster reminder on the dose date. The
+ * dose date is a calendar date stored at UTC midnight, which is the evening
+ * before west of UTC, so the next booster came due a day early there.
+ */
+import { describe, expect, it, vi } from "vitest";
+
+import type { Prisma } from "@/generated/prisma/client";
+import { satisfyBoostersForDose } from "@/lib/vaccinations/satisfy-matching";
+import { shiftDateKey, userDayKey } from "@/lib/tz/format";
+import { startOfLocalDayKey } from "@/lib/tz/local-day";
+
+const TZ = "America/New_York";
+
+function fakeTx() {
+  const updates: Array<Record<string, unknown>> = [];
+  const reminder = {
+    id: "reminder-1",
+    userId: "u1",
+    vaccinationAntigen: "tetanus",
+    intervalDays: 3650,
+    rrule: null,
+    anchorDate: new Date("2016-03-10T05:00:00.000Z"),
+    notifyHour: 9,
+    nextDueAt: new Date("2026-03-07T14:00:00.000Z"),
+    lastSatisfiedAt: null,
+    lastSkippedAt: null,
+    createdAt: new Date("2016-03-10T05:00:00.000Z"),
+    endsOn: null,
+  };
+  const tx = {
+    measurementReminder: {
+      findMany: vi.fn(async () => [reminder]),
+      updateMany: vi.fn(async ({ data }: { data: Record<string, unknown> }) => {
+        updates.push(data);
+        return { count: 1 };
+      }),
+    },
+    measurementReminderEvent: { create: vi.fn(async () => ({})) },
+  };
+  return { tx: tx as unknown as Prisma.TransactionClient, updates };
+}
+
+describe("satisfyBoostersForDose", () => {
+  it("counts the next interval from the dose's own date west of UTC", async () => {
+    const { tx, updates } = fakeTx();
+    await satisfyBoostersForDose(
+      tx,
+      "u1",
+      { antigenSlug: "tetanus" },
+      new Date("2026-03-10T00:00:00.000Z"),
+      TZ,
+    );
+    // Ten years of days after 10 March, on the person's calendar.
+    expect(userDayKey(updates[0].nextDueAt as Date, TZ)).toBe(
+      shiftDateKey("2026-03-10", 3650),
+    );
+    expect(updates[0].lastSatisfiedAt).toEqual(
+      startOfLocalDayKey("2026-03-10", TZ),
+    );
+  });
+});
diff --git a/src/lib/vaccinations/satisfy-matching.ts b/src/lib/vaccinations/satisfy-matching.ts
index 4febaa950..ee12ec64e 100644
--- a/src/lib/vaccinations/satisfy-matching.ts
+++ b/src/lib/vaccinations/satisfy-matching.ts
@@ -52,6 +52,8 @@ import {
   satisfyReminder,
   type SatisfiableReminder,
 } from "@/lib/measurement-reminders/satisfy";
+import { dateOnlyKey } from "@/lib/tz/date-only";
+import { startOfLocalDayKey } from "@/lib/tz/local-day";
 import {
   componentsForDose,
   loadCustomVaccineLookup,
@@ -136,6 +138,10 @@ export async function satisfyBoostersForDose(
     return byAntigen !== 0 ? byAntigen : a.id.localeCompare(b.id);
   });
 
+  // The dose date is a calendar date at UTC midnight, the evening before west
+  // of UTC; the reminder engine reads an instant in the person's zone, so it
+  // gets the start of that day there, as the booster mint does.
+  const satisfiedAt = startOfLocalDayKey(dateOnlyKey(occurredAt), timezone);
   const satisfiedReminderIds: string[] = [];
   for (const reminder of ordered) {
     const row: SatisfiableReminder = {
@@ -160,7 +166,7 @@ export async function satisfyBoostersForDose(
       tx as unknown as PrismaClient,
       row,
       timezone,
-      occurredAt,
+      satisfiedAt,
       "vaccination",
     );
     if (result.satisfied) satisfiedReminderIds.push(reminder.id);
diff --git a/tests/integration/vaccination-booster-mint.test.ts b/tests/integration/vaccination-booster-mint.test.ts
index 3386bd2aa..14184ad47 100644
--- a/tests/integration/vaccination-booster-mint.test.ts
+++ b/tests/integration/vaccination-booster-mint.test.ts
@@ -18,6 +18,8 @@ process.env.ENCRYPTION_KEY ??=
   "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef";
 
 import { POST as createVaccination } from "@/app/api/vaccinations/route";
+import { dateOnlyKey } from "@/lib/tz/date-only";
+import { startOfLocalDayKey } from "@/lib/tz/local-day";
 import { POST as mintBooster } from "@/app/api/vaccinations/[id]/booster/route";
 import { GET as listReminders } from "@/app/api/measurement-reminders/route";
 
@@ -221,7 +223,10 @@ describe("the whole activity-aware loop, through real routes", () => {
     const after = await prisma.measurementReminder.findUniqueOrThrow({
       where: { id: reminderId },
     });
-    expect(after.lastSatisfiedAt?.toISOString()).toBe(occurredAt.toISOString());
+    // The start of the dose's calendar day in the person's zone.
+    expect(after.lastSatisfiedAt).toEqual(
+      startOfLocalDayKey(dateOnlyKey(occurredAt), "Europe/Berlin"),
+    );
     expect(after.nextDueAt!.getTime()).toBeGreaterThan(
       before.nextDueAt!.getTime(),
     );
diff --git a/tests/integration/vaccination-booster-satisfy.test.ts b/tests/integration/vaccination-booster-satisfy.test.ts
index f6d1db2a3..489286d86 100644
--- a/tests/integration/vaccination-booster-satisfy.test.ts
+++ b/tests/integration/vaccination-booster-satisfy.test.ts
@@ -24,6 +24,8 @@ process.env.ENCRYPTION_KEY ??=
   "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef";
 
 import { POST as createVaccination } from "@/app/api/vaccinations/route";
+import { dateOnlyKey } from "@/lib/tz/date-only";
+import { startOfLocalDayKey } from "@/lib/tz/local-day";
 
 import { cookieJar, headerJar } from "./mock-next-headers";
 import { getPrismaClient, truncateAllTables } from "./setup";
@@ -132,7 +134,10 @@ describe("a dose satisfies the boosters it answers", () => {
     expect(res.status, JSON.stringify(await res.clone().json())).toBe(201);
 
     const after = await reminderById(reminder.id);
-    expect(after.lastSatisfiedAt?.toISOString()).toBe(occurredAt.toISOString());
+    // The start of the dose's calendar day in the person's zone.
+    expect(after.lastSatisfiedAt).toEqual(
+      startOfLocalDayKey(dateOnlyKey(occurredAt), "Europe/Berlin"),
+    );
     // Re-anchored from the satisfy instant, so the next one lands roughly ten
     // years out rather than staying on the date it was already overdue on.
     expect(after.nextDueAt).not.toBeNull();