diff --git a/src/app/components/calendar-year/calendar-year.ts b/src/app/components/calendar-year/calendar-year.ts
index e5b3a9b..4813176 100644
--- a/src/app/components/calendar-year/calendar-year.ts
+++ b/src/app/components/calendar-year/calendar-year.ts
@@ -272,11 +272,10 @@ export class CalendarYear {
 		const horizon = this.calendarHorizon();
 		if (!horizon) return undefined;
 		if (this.declarationEntry())
-			return this.calendarService.project({
-				placements: [],
-				horizon,
-				declaredDays: this.userInput.declaredDaysSignal(),
-			});
+			return this.calendarService.project(
+				{ placements: [], horizon, declaredDays: this.userInput.declaredDaysSignal() },
+				{ fixedMonths: true },
+			);
 		const plan = this.displayedPlan();
 		return this.calendarService.project(
 			plan ? { ...plan, horizon: plan.horizon ?? horizon } : { placements: [], horizon },
diff --git a/src/app/services/calendar-service.spec.ts b/src/app/services/calendar-service.spec.ts
index 1df7ce3..51a34a3 100644
--- a/src/app/services/calendar-service.spec.ts
+++ b/src/app/services/calendar-service.spec.ts
@@ -216,6 +216,29 @@ describe('CalendarService', () => {
 			]);
 		});
 
+		it('keeps fixed months when an edge declaration widens the projection', () => {
+			const plan = {
+				placements: [],
+				horizon: { start: '2024-07-01', end: '2025-01-04' },
+				declaredDays: [{ date: '2024-07-01', type: 'cp' as const }],
+			};
+			const months = (projection: ReturnType<typeof service.project>) =>
+				projection.months.map(({ monthIndex, year }) => `${year}-${monthIndex}`);
+			expect(months(service.project(plan))[0]).toBe('2024-5');
+			const fixed = service.project(plan, { fixedMonths: true });
+			expect(months(fixed)).toEqual([
+				'2024-6',
+				'2024-7',
+				'2024-8',
+				'2024-9',
+				'2024-10',
+				'2024-11',
+				'2025-0',
+			]);
+			expect(fixed.selections.dayTypes(day('2024-06-30'))).toContain(DayType.WEEKEND);
+			expect(fixed.selections.isStart(day('2024-07-01'))).toBe(false);
+		});
+
 		it('includes the final partial month of a half-open one-year horizon', () => {
 			const { selections } = project({ start: '2024-01-15', end: '2025-01-15' });
 			expect(selections.dayTypes(day('2025-01-04'))).toContain(DayType.WEEKEND);
diff --git a/src/app/services/calendar-service.ts b/src/app/services/calendar-service.ts
index cdf5d28..0ea9091 100644
--- a/src/app/services/calendar-service.ts
+++ b/src/app/services/calendar-service.ts
@@ -72,10 +72,14 @@ export class CalendarService {
 		return { start, end: addPlanningYears(start, 1) };
 	}
 
+	/** `fixedMonths` lists only the plan horizon's months, so editing declarations never adds a month. */
 	project(
 		plan: CalendarPlan & { horizon: PlanningStateSnapshot['horizon'] },
+		{ fixedMonths = false } = {},
 	): CalendarProjection {
 		const horizon = calendarProjectionHorizon(plan)!;
+		const firstMonth = plan.horizon.start.slice(0, 7);
+		const lastMonth = addPlanningDays(plan.horizon.end, -1).slice(0, 7);
 		const first = new Date(`${horizon.start}T00:00:00`);
 		const last = new Date(`${horizon.end}T00:00:00`);
 		last.setDate(last.getDate() - 1);
@@ -92,7 +96,9 @@ export class CalendarService {
 		const cursor = new Date(first);
 		cursor.setDate(1);
 		while (cursor <= last) {
-			months.push({ monthIndex: cursor.getMonth(), year: cursor.getFullYear() });
+			const month = `${cursor.getFullYear()}-${String(cursor.getMonth() + 1).padStart(2, '0')}`;
+			if (!fixedMonths || (month >= firstMonth && month <= lastMonth))
+				months.push({ monthIndex: cursor.getMonth(), year: cursor.getFullYear() });
 			for (const week of CalendarService.weeksInMonth(
 				cursor.getMonth(),
 				cursor.getFullYear(),