diff --git a/src/components/measurement-reminders/vorsorge-dashboard-card.tsx b/src/components/measurement-reminders/vorsorge-dashboard-card.tsx
index cd101028d..50d820432 100644
--- a/src/components/measurement-reminders/vorsorge-dashboard-card.tsx
+++ b/src/components/measurement-reminders/vorsorge-dashboard-card.tsx
@@ -28,40 +28,15 @@ import { ResponsiveSheet } from "@/components/ui/responsive-sheet";
 import { Skeleton } from "@/components/ui/skeleton";
 import { MeasurementForm } from "@/components/measurements/measurement-form";
 import { isScreeningReminderType } from "@/lib/validations/measurement-reminders";
+import { relativeDueKey } from "@/lib/measurement-reminders/due-day";
 import {
   useMeasurementReminders,
   useMeasurementReminderMutations,
   type MeasurementReminder,
 } from "@/hooks/use-measurement-reminders";
 
-const DAY_MS = 24 * 60 * 60 * 1000;
 const SUMMARY_LIMIT = 3;
 
-/**
- * v1.32.36 — returns a FULLY-QUALIFIED i18n key, not a bare tail. A call site
- * that interpolates the tail onto the namespace root anchors the
- * reverse-coverage guard at that bare namespace, which marks every key under
- * it reachable and turns the guard into a no-op for the whole namespace. The
- * guard now refuses that shape outright, so the key is built whole here.
- */
-function relativeDueKey(
-  nextDueAt: string | null,
-  now: number,
-): { key: string; days: number } {
-  if (!nextDueAt) return { key: "measurementReminders.nextDue.none", days: 0 };
-  const deltaDays = Math.round((new Date(nextDueAt).getTime() - now) / DAY_MS);
-  if (deltaDays < 0)
-    return {
-      key: "measurementReminders.overdueByDays",
-      days: Math.abs(deltaDays),
-    };
-  if (deltaDays === 0)
-    return { key: "measurementReminders.nextDue.today", days: 0 };
-  if (deltaDays === 1)
-    return { key: "measurementReminders.nextDue.tomorrow", days: 1 };
-  return { key: "measurementReminders.nextDue.inDays", days: deltaDays };
-}
-
 function resolveLabel(
   reminder: MeasurementReminder,
   t: (key: string) => string,
diff --git a/src/components/measurement-reminders/vorsorge-section.tsx b/src/components/measurement-reminders/vorsorge-section.tsx
index 5a1d697a7..e1a5f945c 100644
--- a/src/components/measurement-reminders/vorsorge-section.tsx
+++ b/src/components/measurement-reminders/vorsorge-section.tsx
@@ -46,7 +46,7 @@ import {
   useDisplayTimezone,
 } from "@/lib/i18n/context";
 import { relativeCalendarDate } from "@/lib/i18n/relative-time";
-import { startOfLocalDayInTz } from "@/lib/tz/local-day";
+import { relativeDueKey } from "@/lib/measurement-reminders/due-day";
 import { cn } from "@/lib/utils";
 import { applyOrder, useModuleListPrefs } from "@/lib/module-list-prefs";
 import { SettingsCardHeader } from "@/components/settings/_card-header";
@@ -143,42 +143,6 @@ const CADENCE_CUSTOM = "custom";
 const CADENCE_RRULE = "rrule";
 const DAY_MS = 24 * 60 * 60 * 1000;
 
-/**
- * v1.32.36 — returns a FULLY-QUALIFIED i18n key, not a bare tail. A call site
- * that interpolates the tail onto the namespace root anchors the
- * reverse-coverage guard at that bare namespace, which marks every key under
- * it reachable and turns the guard into a no-op for the whole namespace. The
- * guard now refuses that shape outright, so the key is built whole here.
- */
-function relativeDueKey(
-  nextDueAt: string | null,
-  now: number,
-): { key: string; days: number } {
-  if (!nextDueAt) return { key: "measurementReminders.nextDue.none", days: 0 };
-  const due = new Date(nextDueAt);
-  if (Number.isNaN(due.getTime()))
-    return { key: "measurementReminders.nextDue.none", days: 0 };
-  // v1.18.9 (recon) — compare CALENDAR-day deltas in the user's local zone,
-  // not a rolling-24h delta. A reminder due at 09:00 viewed the same evening
-  // at 20:00 must still read "heute", and one whose due calendar-day is before
-  // today must read "überfällig" — a raw `(due - now) / DAY_MS` round would
-  // mis-bucket both. Floor each instant to its local day start (host-local zone
-  // = the browser user's zone for this client component) before differencing.
-  const dueDay = startOfLocalDayInTz(due, undefined).getTime();
-  const nowDay = startOfLocalDayInTz(new Date(now), undefined).getTime();
-  const deltaDays = Math.round((dueDay - nowDay) / DAY_MS);
-  if (deltaDays < 0)
-    return {
-      key: "measurementReminders.overdueByDays",
-      days: Math.abs(deltaDays),
-    };
-  if (deltaDays === 0)
-    return { key: "measurementReminders.nextDue.today", days: 0 };
-  if (deltaDays === 1)
-    return { key: "measurementReminders.nextDue.tomorrow", days: 1 };
-  return { key: "measurementReminders.nextDue.inDays", days: deltaDays };
-}
-
 /**
  * v1.18.1 — a COACH-minted reminder stores an i18n KEY in `label` (the
  * cadence preset's `labelKey`), not free prose. Resolve it through `t()`;
diff --git a/src/lib/measurement-reminders/due-day.ts b/src/lib/measurement-reminders/due-day.ts
new file mode 100644
index 000000000..be07a84db
--- /dev/null
+++ b/src/lib/measurement-reminders/due-day.ts
@@ -0,0 +1,62 @@
+/**
+ * The one place a Vorsorge reminder's next-due instant becomes the words a
+ * person reads: "heute" / "morgen" / "in N Tagen" / "überfällig seit N Tagen".
+ *
+ * It used to live inside the two surfaces that render it — the `/checkups`
+ * page and the dashboard summary card — as two hand-copied bodies. The page
+ * copy was fixed to compare CALENDAR days; the dashboard copy was not, and
+ * kept differencing raw instants. So the same reminder read "morgen" on one
+ * screen and "in 2 Tagen" on the other, and a reminder due this morning
+ * dropped out of "heute" over the course of the afternoon. One body here, both
+ * surfaces importing it, and the two cannot disagree again.
+ *
+ * Sits beside `scheduling.ts` (which computes the server-authoritative
+ * `nextDueAt`) because this is the read side of the same value. Nothing in
+ * this file touches the database or the network, so a client component can
+ * import it directly.
+ */
+import { startOfLocalDayInTz } from "@/lib/tz/local-day";
+
+const DAY_MS = 24 * 60 * 60 * 1000;
+
+/** A fully-qualified i18n key plus the day count it interpolates. */
+export interface RelativeDue {
+  key: string;
+  days: number;
+}
+
+/**
+ * v1.32.36 — returns a FULLY-QUALIFIED i18n key, not a bare tail. A call site
+ * that interpolates the tail onto the namespace root anchors the
+ * reverse-coverage guard at that bare namespace, which marks every key under
+ * it reachable and turns the guard into a no-op for the whole namespace. The
+ * guard now refuses that shape outright, so the key is built whole here.
+ */
+export function relativeDueKey(
+  nextDueAt: string | null,
+  now: number,
+): RelativeDue {
+  if (!nextDueAt) return { key: "measurementReminders.nextDue.none", days: 0 };
+  const due = new Date(nextDueAt);
+  if (Number.isNaN(due.getTime()))
+    return { key: "measurementReminders.nextDue.none", days: 0 };
+  // v1.18.9 (recon) — compare CALENDAR-day deltas in the user's local zone,
+  // not a rolling-24h delta. A reminder due at 09:00 viewed the same evening
+  // at 20:00 must still read "heute", and one whose due calendar-day is before
+  // today must read "überfällig" — a raw `(due - now) / DAY_MS` round would
+  // mis-bucket both. Floor each instant to its local day start (host-local zone
+  // = the browser user's zone for this client component) before differencing.
+  const dueDay = startOfLocalDayInTz(due, undefined).getTime();
+  const nowDay = startOfLocalDayInTz(new Date(now), undefined).getTime();
+  const deltaDays = Math.round((dueDay - nowDay) / DAY_MS);
+  if (deltaDays < 0)
+    return {
+      key: "measurementReminders.overdueByDays",
+      days: Math.abs(deltaDays),
+    };
+  if (deltaDays === 0)
+    return { key: "measurementReminders.nextDue.today", days: 0 };
+  if (deltaDays === 1)
+    return { key: "measurementReminders.nextDue.tomorrow", days: 1 };
+  return { key: "measurementReminders.nextDue.inDays", days: deltaDays };
+}