diff --git a/src/features/online-board/api.test.ts b/src/features/online-board/api.test.ts
index 48396cc5..14aabd5f 100644
--- a/src/features/online-board/api.test.ts
+++ b/src/features/online-board/api.test.ts
@@ -105,7 +105,10 @@ describe("searchFlights", () => {
     expect(url.searchParams.get("arrival")).toBe("JFK");
   });
 
-  it("includes timeFrom and timeTo when provided", async () => {
+  // TIRREDESIGN-11: the backend ignores the raw 4-digit HHMM and only
+  // honours HH:MM:SS. Convert at the API layer (Angular parity —
+  // ApiFormatterService.formatTime). '2400' collapses to '23:59:59'.
+  it("normalises timeFrom/timeTo to HH:MM:SS before sending", async () => {
     const { client, mockFetch } = createMockClient(BOARD_RESPONSE);
 
     await searchFlights(client, {
@@ -116,8 +119,23 @@ describe("searchFlights", () => {
     });
 
     const url = extractUrl(mockFetch);
-    expect(url.searchParams.get("timeFrom")).toBe("0800");
-    expect(url.searchParams.get("timeTo")).toBe("2000");
+    expect(url.searchParams.get("timeFrom")).toBe("08:00:00");
+    expect(url.searchParams.get("timeTo")).toBe("20:00:00");
+  });
+
+  it("collapses timeTo='2400' to '23:59:59'", async () => {
+    const { client, mockFetch } = createMockClient(BOARD_RESPONSE);
+
+    await searchFlights(client, {
+      dateFrom: "20250115",
+      dateTo: "20250116",
+      timeFrom: "0000",
+      timeTo: "2400",
+    });
+
+    const url = extractUrl(mockFetch);
+    expect(url.searchParams.get("timeFrom")).toBe("00:00:00");
+    expect(url.searchParams.get("timeTo")).toBe("23:59:59");
   });
 
   it("omits optional params when not provided", async () => {
diff --git a/src/features/online-board/api.ts b/src/features/online-board/api.ts
index 6234a877..7a2cc306 100644
--- a/src/features/online-board/api.ts
+++ b/src/features/online-board/api.ts
@@ -59,6 +59,21 @@ export interface CalendarParams {
 // API functions
 // ---------------------------------------------------------------------------
 
+/**
+ * Convert a 4-digit 'HHMM' string to the backend's expected 'HH:MM:SS'
+ * form (Angular parity — ApiFormatterService.formatTime).
+ *
+ * TIRREDESIGN-11: the URL carries the slider range as '1000'/'1200' but
+ * the board endpoint ignores numeric 'HHMM' values; it only honours
+ * 'HH:MM:SS'. '2400' collapses to '23:59:59' since midnight of the next
+ * day isn't representable in the format.
+ */
+function formatApiTime(hhmm: string | undefined): string | undefined {
+  if (!hhmm || hhmm.length !== 4) return undefined;
+  if (hhmm === "2400") return "23:59:59";
+  return `${hhmm.slice(0, 2)}:${hhmm.slice(2, 4)}:00`;
+}
+
 /**
  * Search flights on the online board.
  * Maps to: `GET /board?flightNumber=...&dateFrom=...&dateTo=...&...`
@@ -76,8 +91,10 @@ export async function searchFlights(
   if (params.flightNumber) query["flightNumber"] = params.flightNumber;
   if (params.departure) query["departure"] = params.departure;
   if (params.arrival) query["arrival"] = params.arrival;
-  if (params.timeFrom) query["timeFrom"] = params.timeFrom;
-  if (params.timeTo) query["timeTo"] = params.timeTo;
+  const tf = formatApiTime(params.timeFrom);
+  const tt = formatApiTime(params.timeTo);
+  if (tf) query["timeFrom"] = tf;
+  if (tt) query["timeTo"] = tt;
 
   return client.get<IBoardResponse>(`flights/v1.1/${client.locale}/board`, query, signal);
 }
diff --git a/src/features/online-board/components/OnlineBoardSearchPage.tsx b/src/features/online-board/components/OnlineBoardSearchPage.tsx
index c01e3c5c..f0e0a8bb 100644
--- a/src/features/online-board/components/OnlineBoardSearchPage.tsx
+++ b/src/features/online-board/components/OnlineBoardSearchPage.tsx
@@ -128,12 +128,24 @@ function toSearchParams(
 
 /**
  * Convert parsed params into calendar API params.
+ *
+ * TIRREDESIGN-8: the 31-day availability bitmask is always anchored to
+ * today (Angular parity — `updateCalendar()` builds `date = new Date();
+ * date.setDate(date.getDate()-1)` regardless of the URL-selected day).
+ * Using `params.date` as the anchor shifts the window forward as the
+ * user navigates and causes DayTabs inside the -1/+14 range to fall
+ * outside the returned bitmask, grey-listing them even when flights run
+ * that day.
  */
 function toCalendarParams(
   params: OnlineBoardSearchPageProps["params"],
 ): CalendarParams {
+  const today = new Date();
+  const y = today.getFullYear().toString();
+  const m = (today.getMonth() + 1).toString().padStart(2, "0");
+  const d = today.getDate().toString().padStart(2, "0");
   const base: CalendarParams = {
-    date: formatDateForApi(params.date),
+    date: `${y}-${m}-${d}T00:00:00`,
     searchType: params.type as FlightRequestType,
   };
 
diff --git a/src/features/schedule/api.ts b/src/features/schedule/api.ts
index 448837e2..6417c2a4 100644
--- a/src/features/schedule/api.ts
+++ b/src/features/schedule/api.ts
@@ -42,8 +42,13 @@ export async function searchSchedule(
     dateTo: addOneDayIso(params.dateTo),
     connections: String(params.connections ?? 1),
   };
-  if (params.timeFrom) query["timeFrom"] = params.timeFrom;
-  if (params.timeTo) query["timeTo"] = params.timeTo;
+  // TIRREDESIGN-11: the slider carries its value as four-digit 'HHMM' but
+  // the endpoint only honours the colon-delimited 'HH:MM:SS' form (Angular's
+  // ApiFormatterService.formatTime). '2400' collapses to '23:59:59'.
+  const tf = formatApiTime(params.timeFrom);
+  const tt = formatApiTime(params.timeTo);
+  if (tf) query["timeFrom"] = tf;
+  if (tt) query["timeTo"] = tt;
   if (params.attribute) query["attribute"] = String(params.attribute);
   return client.get<IScheduleResponse>(
     `flights/1/${client.locale}/schedule`,
@@ -52,6 +57,12 @@ export async function searchSchedule(
   );
 }
 
+function formatApiTime(hhmm: string | undefined): string | undefined {
+  if (!hhmm || hhmm.length !== 4) return undefined;
+  if (hhmm === "2400") return "23:59:59";
+  return `${hhmm.slice(0, 2)}:${hhmm.slice(2, 4)}:00`;
+}
+
 /** Return the date one day after a yyyy-MM-dd string (passthrough on bad input). */
 function addOneDayIso(iso: string): string {
   const m = /^(\d{4})-(\d{2})-(\d{2})(.*)$/.exec(iso);