diff --git a/src/ui/components/PurchaseDialog.tsx b/src/ui/components/PurchaseDialog.tsx
index 4e3d8e2..ff5b76c 100644
--- a/src/ui/components/PurchaseDialog.tsx
+++ b/src/ui/components/PurchaseDialog.tsx
@@ -37,7 +37,12 @@ import {
   cancelScheduleFrom,
   updatePurchase,
 } from "../../store/actions.ts";
-import { offerableCosts, seedCostFromDraft, sourceFor } from "../recurringForm.ts";
+import {
+  newCostOptionIsSelectable,
+  offerableCosts,
+  seedCostFromDraft,
+  sourceFor,
+} from "../recurringForm.ts";
 import { WEEKDAYS } from "../routes/settings/RecurringSection.tsx";
 import type { Anchoring, MonthId, Purchase, Recurrence } from "../../domain/types.ts";
 
@@ -280,7 +285,21 @@ export function PurchaseDialog({ monthId, purchase, trigger }: Props) {
                     }}
                   />
                   <DateModeToggle
-                    onClick={() => setDraft((d) => ({ ...d, date: d.date.slice(0, 7) }))}
+                    onClick={() => {
+                      const monthOnly = draft.date.slice(0, 7);
+                      setDraft((d) => ({ ...d, date: monthOnly }));
+                      // Dropping to month-only can strand a "+ New recurring
+                      // cost…" selection that needed the day just removed
+                      // (e.g. "days", or "months" anchored to the last
+                      // charge) — its option would show `disabled` below but
+                      // stay selected, so reset to the one pairing that is
+                      // always valid rather than leave state pointing at a
+                      // choice the owner can no longer make.
+                      if (!newCostOptionIsSelectable(newRecurrence.kind, newAnchoring, monthOnly)) {
+                        setNewRecurrence((r) => ({ kind: "everyNMonths", n: r.n }));
+                        setNewAnchoring("calendar");
+                      }
+                    }}
                   >
                     use the month only
                   </DateModeToggle>
@@ -425,9 +444,26 @@ export function PurchaseDialog({ monthId, purchase, trigger }: Props) {
                       );
                     }}
                   >
-                    <option value="everyNDays">days</option>
-                    <option value="everyNWeeks">weeks</option>
-                    <option value="everyNMonths">months</option>
+                    <option
+                      value="everyNDays"
+                      disabled={!newCostOptionIsSelectable("everyNDays", newAnchoring, draft.date)}
+                    >
+                      days
+                    </option>
+                    <option
+                      value="everyNWeeks"
+                      disabled={!newCostOptionIsSelectable("everyNWeeks", newAnchoring, draft.date)}
+                    >
+                      weeks
+                    </option>
+                    <option
+                      value="everyNMonths"
+                      disabled={
+                        !newCostOptionIsSelectable("everyNMonths", newAnchoring, draft.date)
+                      }
+                    >
+                      months
+                    </option>
                   </NativeSelect>
                 </div>
                 {newRecurrence.kind === "everyNWeeks" && (
@@ -463,11 +499,30 @@ export function PurchaseDialog({ monthId, purchase, trigger }: Props) {
                       setNewAnchoring(anchoring);
                     }}
                   >
-                    <option value="calendar">the calendar</option>
-                    <option value="lastCharge">the last charge</option>
+                    <option
+                      value="calendar"
+                      disabled={!newCostOptionIsSelectable(newRecurrence.kind, "calendar", draft.date)}
+                    >
+                      the calendar
+                    </option>
+                    <option
+                      value="lastCharge"
+                      disabled={
+                        !newCostOptionIsSelectable(newRecurrence.kind, "lastCharge", draft.date)
+                      }
+                    >
+                      the last charge
+                    </option>
                   </NativeSelect>
                 </div>
               </div>
+              {draft.date.length <= 7 && (
+                <p className="text-xs text-budget-ink-muted">
+                  Days, weeks, and "the last charge" need an exact date — use
+                  "set exact date" next to the date above, then pick the unit
+                  or anchoring you want.
+                </p>
+              )}
               <p className="text-xs text-budget-ink-muted">
                 This purchase becomes its first charge. Anchored to the last
                 charge, its cycle restarts from today rather than from a fixed
diff --git a/src/ui/recurringForm.test.ts b/src/ui/recurringForm.test.ts
index be4a05e..c6042c5 100644
--- a/src/ui/recurringForm.test.ts
+++ b/src/ui/recurringForm.test.ts
@@ -1,5 +1,10 @@
 import { describe, expect, test } from "bun:test";
-import { offerableCosts, seedCostFromDraft, sourceFor } from "./recurringForm.ts";
+import {
+  newCostOptionIsSelectable,
+  offerableCosts,
+  seedCostFromDraft,
+  sourceFor,
+} from "./recurringForm.ts";
 import { createSeedDataset } from "../domain/seed.ts";
 import * as actions from "../store/actions.ts";
 import type { Dataset } from "../domain/types.ts";
@@ -100,6 +105,40 @@ describe("sourceFor", () => {
   });
 });
 
+// I2 fix: a month-only purchase date can only ever seed "everyNMonths" +
+// "calendar" (rent occurs IN a month). Every other pairing hands the date
+// straight to day-precise arithmetic and would throw a store-level error
+// naming a field ("startDate") the purchase dialog never shows — so the
+// dialog must not OFFER those options while the date is month-only, rather
+// than reword the throw. This predicate is what decides which options are
+// offered; it is tested directly, since a DOM test cannot exercise a
+// predicate the way it can a rendered <option disabled>.
+describe("newCostOptionIsSelectable", () => {
+  test("everyNMonths + calendar is selectable at either granularity — rent occurs IN a month", () => {
+    expect(newCostOptionIsSelectable("everyNMonths", "calendar", "2026-09")).toBe(true);
+    expect(newCostOptionIsSelectable("everyNMonths", "calendar", "2026-09-20")).toBe(true);
+  });
+
+  test("everyNMonths + lastCharge needs a day-granular date", () => {
+    expect(newCostOptionIsSelectable("everyNMonths", "lastCharge", "2026-09")).toBe(false);
+    expect(newCostOptionIsSelectable("everyNMonths", "lastCharge", "2026-09-20")).toBe(true);
+  });
+
+  test("everyNDays always needs a day-granular date, whichever anchoring", () => {
+    expect(newCostOptionIsSelectable("everyNDays", "calendar", "2026-09")).toBe(false);
+    expect(newCostOptionIsSelectable("everyNDays", "lastCharge", "2026-09")).toBe(false);
+    expect(newCostOptionIsSelectable("everyNDays", "calendar", "2026-09-20")).toBe(true);
+    expect(newCostOptionIsSelectable("everyNDays", "lastCharge", "2026-09-20")).toBe(true);
+  });
+
+  test("everyNWeeks always needs a day-granular date, whichever anchoring", () => {
+    expect(newCostOptionIsSelectable("everyNWeeks", "calendar", "2026-09")).toBe(false);
+    expect(newCostOptionIsSelectable("everyNWeeks", "lastCharge", "2026-09")).toBe(false);
+    expect(newCostOptionIsSelectable("everyNWeeks", "calendar", "2026-09-20")).toBe(true);
+    expect(newCostOptionIsSelectable("everyNWeeks", "lastCharge", "2026-09-20")).toBe(true);
+  });
+});
+
 describe("seedCostFromDraft", () => {
   test("carries the purchase's amount, currency, splits and date", () => {
     const draft = {
diff --git a/src/ui/recurringForm.ts b/src/ui/recurringForm.ts
index 355c68c..1426db1 100644
--- a/src/ui/recurringForm.ts
+++ b/src/ui/recurringForm.ts
@@ -79,6 +79,33 @@ export function sourceFor(
  * day-precisely from the charge, and the two day-granular recurrence kinds
  * hand their `startDate` straight to `addDays` regardless of anchoring.
  */
+/**
+ * Whether a NEW cost's kind+anchoring pairing can be seeded from a purchase
+ * dated `purchaseDate`.
+ *
+ * `seedCostFromDraft` hands `purchaseDate` straight to `startDate` whenever
+ * the pairing is not month-granular, so this mirrors
+ * `requireStartDateGranularity`'s `needsDay` in `store/actions.ts`: every
+ * pairing needs a real day EXCEPT "everyNMonths" anchored to "calendar" —
+ * rent occurs IN a month, so a month-only date is fine there.
+ *
+ * A month-only `purchaseDate` reaching any OTHER pairing would reach the
+ * store's own validation and throw text naming a field ("startDate") the
+ * purchase dialog never shows to begin with — so the control offering these
+ * choices must not offer one that always fails; it disables it instead
+ * (PurchaseDialog's "+ New recurring cost…" unit and anchoring selects).
+ * Exported so each option can check this without duplicating the rule, and so
+ * it is testable on its own — a DOM test cannot exercise a predicate.
+ */
+export function newCostOptionIsSelectable(
+  kind: Recurrence["kind"],
+  anchoring: Anchoring,
+  purchaseDate: IsoDate,
+): boolean {
+  const needsDay = kind !== "everyNMonths" || anchoring === "lastCharge";
+  return !needsDay || purchaseDate.length > 7;
+}
+
 export function seedCostFromDraft(
   draft: PurchaseDraft,
   recurrence: Recurrence,