diff --git a/src/domain/months.test.ts b/src/domain/months.test.ts
index b04d05e..119b171 100644
--- a/src/domain/months.test.ts
+++ b/src/domain/months.test.ts
@@ -108,3 +108,12 @@ test("monthOf still rejects out-of-range and malformed month-only values", () =>
   expect(() => monthOf("2026-00")).toThrow(/2026-00/);
   expect(() => monthOf("2026-9")).toThrow(/2026-9/);
 });
+
+// Residual C1: the regex used to be `/^(\d{4})-(\d{2})/` with no trailing
+// anchor, so it matched a "YYYY-MM" PREFIX of a longer string and silently
+// dropped the rest — "2026-091" read back as September 2026. Such a value
+// would then be un-re-importable, since PURCHASE_DATE (src/export/json.ts)
+// rejects it outright.
+test("monthOf rejects a value that is only a prefix match, not a full one", () => {
+  expect(() => monthOf("2026-091")).toThrow(/2026-091/);
+});
diff --git a/src/domain/months.ts b/src/domain/months.ts
index 6fb0e2c..ddccf7c 100644
--- a/src/domain/months.ts
+++ b/src/domain/months.ts
@@ -23,8 +23,14 @@ function fromOrdinal(ordinal: number): MonthId {
   return format(Math.floor(ordinal / 12), (ordinal % 12) + 1);
 }
 
+// Anchored at both ends: "YYYY-MM" or "YYYY-MM-DD" only, never a prefix of
+// something longer. Dual acceptance (with or without a day) is deliberate and
+// load-bearing — do not narrow it to one form. An unanchored match once let
+// "2026-091" through as September 2026, silently dropping the trailing "1".
+const MONTH_OF_DATE = /^(\d{4})-(\d{2})(?:-\d{2})?$/;
+
 export function monthOf(date: IsoDate): MonthId {
-  const match = /^(\d{4})-(\d{2})/.exec(date);
+  const match = MONTH_OF_DATE.exec(date);
   if (!match) throw new Error(`Invalid IsoDate: ${date}`);
   const month = Number(match[2]);
   if (month < 1 || month > 12) throw new Error(`Invalid IsoDate: ${date}`);
diff --git a/src/export/json.test.ts b/src/export/json.test.ts
index ce87074..4e436d7 100644
--- a/src/export/json.test.ts
+++ b/src/export/json.test.ts
@@ -348,6 +348,43 @@ describe("recurring cost validation", () => {
     ).toBe(1);
   });
 
+  // Residual C1: PURCHASE_DATE/DAY_GRANULAR_DATE check shape and range
+  // ("31" is a valid day-of-month digit pair), not calendar validity.
+  // September has 30 days, so "2026-09-31" passed both regexes and the
+  // fold later threw "Invalid day in IsoDate" out of addDays/toDayOrdinal.
+  test("an impossible day on a day-granular kind is refused, not just shape-checked (residual C1)", () => {
+    expect(() =>
+      parseDatasetJson(
+        withRecurring([
+          { ...valid, startDate: "2026-02-30", recurrence: { kind: "everyNDays", n: 1 } },
+        ]),
+      ),
+    ).toThrow(/calendar-impossible/);
+  });
+
+  test("an impossible day is refused even under everyNMonths (residual C1)", () => {
+    expect(() =>
+      parseDatasetJson(withRecurring([{ ...valid, startDate: "2026-09-31" }])),
+    ).toThrow(/calendar-impossible/);
+  });
+
+  test("leap-year and non-leap-year Februaries are accepted (residual C1)", () => {
+    expect(
+      parseDatasetJson(
+        withRecurring([
+          { ...valid, startDate: "2024-02-29", recurrence: { kind: "everyNDays", n: 1 } }, // 2024 is a leap year
+        ]),
+      ).recurring.length,
+    ).toBe(1);
+    expect(
+      parseDatasetJson(
+        withRecurring([
+          { ...valid, startDate: "2026-02-28", recurrence: { kind: "everyNDays", n: 1 } }, // 2026 is not
+        ]),
+      ).recurring.length,
+    ).toBe(1);
+  });
+
   test("an unknown recurrence kind is refused", () => {
     expect(() => parseDatasetJson(withRecurring([{ ...valid, recurrence: { kind: "everyFullMoon", n: 1 } }])))
       .toThrow(/unknown kind/);
diff --git a/src/export/json.ts b/src/export/json.ts
index babaf50..3f6c17b 100644
--- a/src/export/json.ts
+++ b/src/export/json.ts
@@ -1,5 +1,7 @@
 import type { Currency, CurrencyDef, Dataset, MonthId } from "../domain/types.ts";
 import { migrate } from "../store/migrations.ts";
+import { monthOf } from "../domain/months.ts";
+import { toDayOrdinal } from "../domain/days.ts";
 
 export class ImportValidationError extends Error {
   constructor(message: string) {
@@ -227,9 +229,10 @@ function requireSplits(
  * split points at a real post, every purchase has exactly one remainder
  * absorber, every MonthId parses, every rule version resolves unambiguously
  * to a number rather than to NaN or a missing rate, and every recurring
- * cost's startDate is granular enough for its recurrence kind (day-granular
- * for everyNDays/everyNWeeks, since occurrencesOf hands it straight to
- * addDays).
+ * cost's startDate is both granular enough for its recurrence kind
+ * (day-granular for everyNDays/everyNWeeks, since occurrencesOf hands it
+ * straight to addDays) and calendar-valid (no "2026-09-31"), since shape and
+ * range alone let an impossible date through.
  */
 export function parseDatasetJson(text: string): Dataset {
   let raw: unknown;
@@ -344,11 +347,25 @@ export function parseDatasetJson(text: string): Dataset {
     // granularity; the other two kinds reach `addDays` and need a
     // day-granular startDate specifically. `endedFrom` needs no such check —
     // it is only ever compared lexicographically, never fed to `addDays`.
-    if (cost.recurrence.kind !== "everyNMonths" && !DAY_GRANULAR_DATE.test(cost.startDate)) {
+    const dayGranular = DAY_GRANULAR_DATE.test(cost.startDate);
+    if (cost.recurrence.kind !== "everyNMonths" && !dayGranular) {
       throw new ImportValidationError(
         `${label} has a "${cost.recurrence.kind}" recurrence but a month-only start date "${cost.startDate}"; it needs "YYYY-MM-DD"`,
       );
     }
+    // Shape and range (checked above by PURCHASE_DATE/DAY_GRANULAR_DATE) still
+    // let a calendar-impossible date through, e.g. "2026-09-31" (September has
+    // 30 days) or "2026-02-30". Route it through the same domain parsers the
+    // fold itself uses, so an impossible date is refused here instead of
+    // throwing three months later out of `foldBalances`.
+    try {
+      if (dayGranular) toDayOrdinal(cost.startDate);
+      else monthOf(cost.startDate);
+    } catch (error) {
+      throw new ImportValidationError(
+        `${label} has a calendar-impossible start date "${cost.startDate}" (${(error as Error).message})`,
+      );
+    }
 
     if (!ANCHORINGS.has(cost.anchoring)) {
       throw new ImportValidationError(
diff --git a/src/store/actions.test.ts b/src/store/actions.test.ts
index 8c97ddd..30b9f44 100644
--- a/src/store/actions.test.ts
+++ b/src/store/actions.test.ts
@@ -593,6 +593,76 @@ describe("recurring costs", () => {
       expect(data.recurring[0]!.startDate).toBe("2026-09-01");
       expect(data.recurring[0]!.recurrence.kind).toBe("everyNDays");
     });
+
+    // Residual C1: shape alone ("YYYY-MM-DD") is not calendar validity.
+    // September has 30 days, so "2026-09-31" is day-SHAPED but impossible.
+    // `requireStartDateGranularity` used a shape-only regex and let this
+    // through even under everyNMonths; downstream, foldBalances throws
+    // "Invalid day in IsoDate" out of addDays/toDayOrdinal.
+    test("a calendar-impossible startDate is refused, even under everyNMonths", () => {
+      const data = draft();
+      expect(() =>
+        actions.addRecurringCost(data, { ...rentInput, startDate: "2026-09-31" }),
+      ).toThrow(/2026-09-31/);
+
+      const cost = actions.addRecurringCost(data, rentInput);
+      expect(() =>
+        actions.updateRecurringCost(data, cost.id, { startDate: "2026-09-31" }),
+      ).toThrow(/2026-09-31/);
+      // Refused, so nothing committed beyond the one successful add above.
+      expect(data.recurring.length).toBe(1);
+      expect(data.recurring[0]!.startDate).toBe(rentInput.startDate);
+    });
+
+    // Second half of the same reproduction: even if a calendar-impossible
+    // startDate is already sitting in the draft (e.g. from data written
+    // before this fix existed), switching the recurrence kind must not wave
+    // it through — requireStartDateGranularity re-validates on every update,
+    // not only ones that touch startDate.
+    test("switching kind against an already-impossible stored startDate is refused", () => {
+      const data = draft();
+      const cost = actions.addRecurringCost(data, rentInput);
+      // Bypass the public API to simulate a bad value already on disk.
+      data.recurring[0]!.startDate = "2026-09-31";
+      expect(() =>
+        actions.updateRecurringCost(data, cost.id, { recurrence: { kind: "everyNDays", n: 7 } }),
+      ).toThrow(/2026-09-31/);
+      expect(data.recurring[0]!.recurrence.kind).toBe("everyNMonths");
+    });
+
+    // Residual C1: monthOf's regex used to be unanchored (`/^(\d{4})-(\d{2})/`
+    // with no trailing `$`), so it matched a "YYYY-MM" PREFIX of a longer,
+    // malformed string and silently dropped the rest.
+    test("an unanchored-month-shaped garbage value is refused", () => {
+      const data = draft();
+      expect(() =>
+        actions.addRecurringCost(data, { ...rentInput, startDate: "2026-091" }),
+      ).toThrow(/2026-091/);
+
+      const cost = actions.addRecurringCost(data, rentInput);
+      expect(() =>
+        actions.updateRecurringCost(data, cost.id, { startDate: "2026-091" }),
+      ).toThrow(/2026-091/);
+    });
+
+    test("legitimate calendar-valid dates still pass", () => {
+      const data = draft();
+      expect(() => actions.addRecurringCost(data, { ...rentInput, startDate: "2026-09" })).not.toThrow();
+      expect(() =>
+        actions.addRecurringCost(data, {
+          ...rentInput,
+          startDate: "2024-02-29", // 2024 is a leap year
+          recurrence: { kind: "everyNDays", n: 1 },
+        }),
+      ).not.toThrow();
+      expect(() =>
+        actions.addRecurringCost(data, {
+          ...rentInput,
+          startDate: "2026-02-28", // 2026 is not a leap year
+          recurrence: { kind: "everyNDays", n: 1 },
+        }),
+      ).not.toThrow();
+    });
   });
 
   test("ending sets both endedFrom and archived", () => {
diff --git a/src/store/actions.ts b/src/store/actions.ts
index edbefe9..0ef951f 100755
--- a/src/store/actions.ts
+++ b/src/store/actions.ts
@@ -1,5 +1,6 @@
 import { newId } from "../domain/seed.ts";
 import { compareMonths, monthOf } from "../domain/months.ts";
+import { toDayOrdinal } from "../domain/days.ts";
 import { roundMoney } from "../domain/money.ts";
 import { currencyUsage, normalizeCurrencyCode } from "../domain/currencies.ts";
 import type {
@@ -402,14 +403,29 @@ const DAY_GRANULAR_DATE = /^\d{4}-\d{2}-\d{2}$/;
  * to `addDays`, which throws on a month-only date — so those kinds require a
  * day-granular `startDate` here, at the boundary, rather than three months
  * later when a fold walks off the end of it.
+ *
+ * Shape alone is not enough: "2026-09-31" is day-SHAPED but calendar-
+ * impossible (September has 30 days), and would otherwise reach `addDays` (or
+ * sit unnoticed under `everyNMonths`) and throw three months later out of the
+ * fold instead of here. Route a day-granular value through `toDayOrdinal` and
+ * a month-granular one through `monthOf` — both already throw on exactly
+ * that — and translate whatever they throw into this file's own error style.
  */
 function requireStartDateGranularity(recurrence: Recurrence, startDate: IsoDate): void {
-  if (recurrence.kind === "everyNMonths") return;
-  if (!DAY_GRANULAR_DATE.test(startDate)) {
+  const dayGranular = DAY_GRANULAR_DATE.test(startDate);
+  if (recurrence.kind !== "everyNMonths" && !dayGranular) {
     throw new Error(
       `A "${recurrence.kind}" recurrence needs a day-granular startDate ("YYYY-MM-DD"), not "${startDate}"`,
     );
   }
+  try {
+    if (dayGranular) toDayOrdinal(startDate);
+    else monthOf(startDate);
+  } catch (error) {
+    throw new Error(
+      `A "${recurrence.kind}" recurrence needs a valid startDate, not "${startDate}" (${(error as Error).message})`,
+    );
+  }
 }
 
 export function addRecurringCost(
diff --git a/src/ui/routes/settings/RecurringSection.test.ts b/src/ui/routes/settings/RecurringSection.test.ts
index 4b011b9..e6f3dad 100644
--- a/src/ui/routes/settings/RecurringSection.test.ts
+++ b/src/ui/routes/settings/RecurringSection.test.ts
@@ -36,6 +36,20 @@ describe("isValidStartDate", () => {
   test("a day-granular kind rejects a malformed day-granular-shaped value", () => {
     expect(isValidStartDate("2026-02-30", "everyNDays")).toBe(false); // no Feb 30
   });
+
+  // Residual C1: `monthOf` only extracts the month and never checks the day,
+  // so a day-SHAPED but calendar-impossible value ("2026-09-31" — September
+  // has 30 days) used to pass here even under everyNMonths, and could then be
+  // stored, only for foldBalances to throw three months later.
+  test("everyNMonths still rejects a day-granular value with an impossible day", () => {
+    expect(isValidStartDate("2026-09-31", "everyNMonths")).toBe(false);
+  });
+
+  test("everyNMonths accepts a real leap-day and rejects a non-leap Feb 29", () => {
+    expect(isValidStartDate("2024-02-29", "everyNMonths")).toBe(true); // 2024 is a leap year
+    expect(isValidStartDate("2026-02-29", "everyNMonths")).toBe(false); // 2026 is not
+    expect(isValidStartDate("2026-02-28", "everyNMonths")).toBe(true);
+  });
 });
 
 // C1: switching the unit dropdown to a day-granular kind must expand a
diff --git a/src/ui/routes/settings/RecurringSection.tsx b/src/ui/routes/settings/RecurringSection.tsx
index ae8c7c8..fbaba4f 100644
--- a/src/ui/routes/settings/RecurringSection.tsx
+++ b/src/ui/routes/settings/RecurringSection.tsx
@@ -32,6 +32,12 @@ export function isDayGranular(date: IsoDate): boolean {
  * `addDays` directly. Reuses the domain's own parsers rather than a second,
  * possibly-diverging regex.
  *
+ * A day-SHAPED value is routed through `toDayOrdinal` even under
+ * `everyNMonths`: `monthOf` only extracts the month and never checks the day,
+ * so without this a calendar-impossible date like "2026-09-31" (September has
+ * 30 days) would pass here and could still be stored, only to throw three
+ * months later out of the fold.
+ *
  * Exported for `RecurringSection.test.ts` — this is the one thing standing
  * between the "Starts" field and C2 (an empty or malformed value bricking
  * every route the fold touches), so it is tested directly rather than only
@@ -39,7 +45,7 @@ export function isDayGranular(date: IsoDate): boolean {
  */
 export function isValidStartDate(value: string, kind: Recurrence["kind"]): boolean {
   try {
-    if (kind === "everyNMonths") monthOf(value);
+    if (kind === "everyNMonths" && !isDayGranular(value)) monthOf(value);
     else toDayOrdinal(value);
     return true;
   } catch {