diff --git a/src/features/schedule/components/ScheduleFilter.tsx b/src/features/schedule/components/ScheduleFilter.tsx
index a658a87f..b5ee7de9 100644
--- a/src/features/schedule/components/ScheduleFilter.tsx
+++ b/src/features/schedule/components/ScheduleFilter.tsx
@@ -79,9 +79,22 @@ function todayIso(): string {
   return `${y}-${m}-${day}`;
 }
 
+/**
+ * Format a Date as `yyyy-MM-dd` — the shape returned by the schedule
+ * `/calendar` API (`bitmaskToDates` in api.ts). Used only for the
+ * disabled-dates set lookup; the URL builder uses the dashless
+ * `dateToYyyymmdd` form above.
+ */
+function dateToIsoYmd(value: Date): string {
+  const y = value.getFullYear().toString();
+  const m = (value.getMonth() + 1).toString().padStart(2, "0");
+  const d = value.getDate().toString().padStart(2, "0");
+  return `${y}-${m}-${d}`;
+}
+
 /** Inverse of the API's enabled-days list: every date inside
- *  [minDate, maxDate] that isn't in `availableYmd` gets disabled so
- *  PrimeReact's Calendar greys them out (TIRREDESIGN-12). */
+ *  [minDate, maxDate] that isn't in `availableYmd` (yyyy-MM-dd) gets
+ *  disabled so PrimeReact's Calendar greys them out (TIRREDESIGN-12). */
 function computeDisabledDates(
   availableYmd: string[],
   minDate: Date,
@@ -92,7 +105,7 @@ function computeDisabledDates(
   const cursor = new Date(minDate);
   cursor.setHours(0, 0, 0, 0);
   while (cursor.getTime() <= maxDate.getTime()) {
-    const ymd = dateToYyyymmdd(cursor);
+    const ymd = dateToIsoYmd(cursor);
     if (!available.has(ymd)) {
       out.push(new Date(cursor));
     }
diff --git a/tests/e2e/schedule-calendar-operating-days.spec.ts b/tests/e2e/schedule-calendar-operating-days.spec.ts
new file mode 100644
index 00000000..e606e67f
--- /dev/null
+++ b/tests/e2e/schedule-calendar-operating-days.spec.ts
@@ -0,0 +1,47 @@
+import { test, expect } from "@playwright/test";
+
+// TIRREDESIGN-12 — when both schedule cities are filled, the date-picker
+// must grey out the days the route does NOT operate. The fix in
+// ScheduleFilter.tsx aligned the date-format used for the
+// available-days set lookup (yyyy-MM-dd, matching the schedule
+// `/days` API output) — previously the lookup compared `yyyymmdd`
+// against `yyyy-MM-dd`, so every day was treated as unavailable
+// and the entire calendar greyed out.
+
+test("Schedule calendar greys out non-operating days for the route", async ({
+  page,
+}) => {
+  await page.goto("/ru-ru/schedule/route/MOW-MMK-20260427-20260503");
+  await expect(page.locator(".day-grouped-flight-list").first()).toBeVisible({
+    timeout: 15000,
+  });
+
+  // Open the picker.
+  await page.locator("button.p-datepicker-trigger").first().click();
+  const panel = page.locator(".p-datepicker-panel, .p-datepicker").first();
+  await expect(panel).toBeVisible();
+
+  // Wait for the operating-days API to come back (the disabled-set
+  // is derived from it; before that, every day is enabled).
+  await expect
+    .poll(async () =>
+      page
+        .locator(".p-datepicker td span.p-disabled")
+        .count(),
+    { timeout: 10000 })
+    .toBeGreaterThan(0);
+
+  // The MOW→MMK route operates roughly daily today onward, so the
+  // visible month must contain BOTH enabled and disabled cells.
+  // The check is intentionally loose because the live operating
+  // schedule shifts by route — but a fully-enabled or fully-disabled
+  // calendar would prove the format-mismatch regression returned.
+  const enabled = await page
+    .locator(".p-datepicker td span:not(.p-disabled)")
+    .count();
+  const disabled = await page
+    .locator(".p-datepicker td span.p-disabled")
+    .count();
+  expect(enabled).toBeGreaterThan(0);
+  expect(disabled).toBeGreaterThan(0);
+});