diff --git a/src/store/actions.test.ts b/src/store/actions.test.ts
index 59620b6..8c97ddd 100644
--- a/src/store/actions.test.ts
+++ b/src/store/actions.test.ts
@@ -515,6 +515,86 @@ describe("recurring costs", () => {
     ).toThrow(/at least 1/);
   });
 
+  describe("startDate granularity (C1)", () => {
+    test("adding a day-granular kind with a month-only startDate is refused", () => {
+      const data = draft();
+      expect(() =>
+        actions.addRecurringCost(data, {
+          ...rentInput,
+          startDate: "2026-01",
+          recurrence: { kind: "everyNDays", n: 30 },
+        }),
+      ).toThrow(/day-granular/);
+      expect(() =>
+        actions.addRecurringCost(data, {
+          ...rentInput,
+          startDate: "2026-01",
+          recurrence: { kind: "everyNWeeks", n: 1, weekday: 1 },
+        }),
+      ).toThrow(/day-granular/);
+    });
+
+    test("adding a day-granular kind with a day-granular startDate succeeds", () => {
+      const data = draft();
+      const cost = actions.addRecurringCost(data, {
+        ...rentInput,
+        startDate: "2026-01-05",
+        recurrence: { kind: "everyNDays", n: 30 },
+      });
+      expect(cost.startDate).toBe("2026-01-05");
+    });
+
+    test("everyNMonths accepts either granularity", () => {
+      const data = draft();
+      expect(() => actions.addRecurringCost(data, { ...rentInput, startDate: "2026-01" })).not.toThrow();
+      expect(() =>
+        actions.addRecurringCost(data, { ...rentInput, startDate: "2026-01-15" }),
+      ).not.toThrow();
+    });
+
+    // This is the exact bug: a monthly cost is created (startDate "2026-09"),
+    // then the unit dropdown switches it to a day-granular kind WITHOUT
+    // touching startDate — the only way a real edit reaches this, since the
+    // add form always starts as everyNMonths.
+    test("updating just the recurrence kind against an untouched month-only startDate is refused", () => {
+      const data = draft();
+      const cost = actions.addRecurringCost(data, { ...rentInput, startDate: "2026-09" });
+      expect(() =>
+        actions.updateRecurringCost(data, cost.id, { recurrence: { kind: "everyNDays", n: 30 } }),
+      ).toThrow(/day-granular/);
+      expect(() =>
+        actions.updateRecurringCost(data, cost.id, {
+          recurrence: { kind: "everyNWeeks", n: 1, weekday: 3 },
+        }),
+      ).toThrow(/day-granular/);
+      // Refused, so nothing committed — still the original month-granular kind.
+      expect(data.recurring[0]!.recurrence.kind).toBe("everyNMonths");
+    });
+
+    test("updating just startDate to a month-only value against a stored day-granular kind is refused", () => {
+      const data = draft();
+      const cost = actions.addRecurringCost(data, {
+        ...rentInput,
+        startDate: "2026-01-05",
+        recurrence: { kind: "everyNDays", n: 30 },
+      });
+      expect(() => actions.updateRecurringCost(data, cost.id, { startDate: "2026-01" })).toThrow(
+        /day-granular/,
+      );
+    });
+
+    test("updating recurrence and startDate together to a consistent day-granular pair succeeds", () => {
+      const data = draft();
+      const cost = actions.addRecurringCost(data, { ...rentInput, startDate: "2026-09" });
+      actions.updateRecurringCost(data, cost.id, {
+        recurrence: { kind: "everyNDays", n: 30 },
+        startDate: "2026-09-01",
+      });
+      expect(data.recurring[0]!.startDate).toBe("2026-09-01");
+      expect(data.recurring[0]!.recurrence.kind).toBe("everyNDays");
+    });
+  });
+
   test("ending sets both endedFrom and archived", () => {
     const data = draft();
     const cost = actions.addRecurringCost(data, rentInput);
diff --git a/src/store/actions.ts b/src/store/actions.ts
index 9a3d57e..edbefe9 100755
--- a/src/store/actions.ts
+++ b/src/store/actions.ts
@@ -394,15 +394,35 @@ function requireRecurrence(recurrence: Recurrence): Recurrence {
   return recurrence;
 }
 
+const DAY_GRANULAR_DATE = /^\d{4}-\d{2}-\d{2}$/;
+
+/**
+ * `everyNMonths` walks the cursor through `monthOf`, so it tolerates either
+ * granularity of `startDate`. `everyNDays` and `everyNWeeks` hand it straight
+ * to `addDays`, which throws on a month-only date — so those kinds require a
+ * day-granular `startDate` here, at the boundary, rather than three months
+ * later when a fold walks off the end of it.
+ */
+function requireStartDateGranularity(recurrence: Recurrence, startDate: IsoDate): void {
+  if (recurrence.kind === "everyNMonths") return;
+  if (!DAY_GRANULAR_DATE.test(startDate)) {
+    throw new Error(
+      `A "${recurrence.kind}" recurrence needs a day-granular startDate ("YYYY-MM-DD"), not "${startDate}"`,
+    );
+  }
+}
+
 export function addRecurringCost(
   draft: Dataset,
   cost: Omit<RecurringCost, "id" | "order">,
 ): RecurringCost {
+  const recurrence = requireRecurrence(cost.recurrence);
+  requireStartDateGranularity(recurrence, cost.startDate);
   const created: RecurringCost = {
     ...cost,
     id: newId(),
     order: draft.recurring.length,
-    recurrence: requireRecurrence(cost.recurrence),
+    recurrence,
     amount: roundMoneyValue(draft, cost.amount),
     splits: roundSplits(draft, cost.splits, cost.splitMode, cost.amount.currency),
   };
@@ -421,6 +441,13 @@ export function updateRecurringCost(
   if (changes.recurrence) {
     resolved.recurrence = requireRecurrence(changes.recurrence);
   }
+  // Runs on every update, not just one that touches recurrence or startDate:
+  // either field alone can break the pairing (a kind flip against an
+  // untouched month-only startDate is exactly how this bug was reached).
+  requireStartDateGranularity(
+    resolved.recurrence ?? cost.recurrence,
+    resolved.startDate ?? cost.startDate,
+  );
   if (changes.amount) {
     resolved.amount = roundMoneyValue(draft, changes.amount);
   }
@@ -454,13 +481,14 @@ export function moveRecurringCost(
 }
 
 /**
- * Stops a bill from `from` onward and hides it from the list.
+ * Stops a bill from `from` onward and dims it in the list.
  *
  * Both fields, because they mean different things: `endedFrom` stops the
- * PROJECTION, and `archived` only hides the row. Archiving alone would leave
- * the bill projecting forever; setting `endedFrom` alone would leave a dead
- * bill cluttering the list. Neither touches a past occurrence, so no
- * historical figure moves.
+ * PROJECTION, and `archived` only flags the row — same as a post, it stays
+ * listed (sorted in, rendered at reduced opacity) rather than being filtered
+ * out. Archiving alone would leave the bill projecting forever; setting
+ * `endedFrom` alone would leave a dead bill undimmed in the list. Neither
+ * touches a past occurrence, so no historical figure moves.
  */
 export function endRecurringCost(draft: Dataset, id: RecurringCostId, from: IsoDate): void {
   const cost = requireRecurringCost(draft, id);