diff --git a/src/features/online-board/timeDefaults.test.ts b/src/features/online-board/timeDefaults.test.ts
new file mode 100644
index 00000000..536925a5
--- /dev/null
+++ b/src/features/online-board/timeDefaults.test.ts
@@ -0,0 +1,43 @@
+import { describe, expect, it } from "vitest";
+import { getOnlineBoardDefaultTimeRange } from "./timeDefaults.js";
+
+describe("4.1.1-R4 + R5: Online-Board default time range", () => {
+  it("desktop/tablet default = 00:00-24:00 regardless of time", () => {
+    expect(getOnlineBoardDefaultTimeRange(false, new Date(2026, 4, 15, 9, 0))).toEqual({
+      timeFrom: "0000",
+      timeTo: "2400",
+    });
+    expect(getOnlineBoardDefaultTimeRange(false, new Date(2026, 4, 15, 22, 30))).toEqual({
+      timeFrom: "0000",
+      timeTo: "2400",
+    });
+  });
+
+  it("mobile default = -1h/+3h from user's current hour", () => {
+    expect(getOnlineBoardDefaultTimeRange(true, new Date(2026, 4, 15, 9, 0))).toEqual({
+      timeFrom: "0800",
+      timeTo: "1200",
+    });
+  });
+
+  it("mobile default clamps to 00:00 when current hour < 1", () => {
+    expect(getOnlineBoardDefaultTimeRange(true, new Date(2026, 4, 15, 0, 30))).toEqual({
+      timeFrom: "0000",
+      timeTo: "0330",
+    });
+  });
+
+  it("mobile default clamps to 24:00 when current hour > 21", () => {
+    expect(getOnlineBoardDefaultTimeRange(true, new Date(2026, 4, 15, 22, 15))).toEqual({
+      timeFrom: "2115",
+      timeTo: "2400",
+    });
+  });
+
+  it("mobile default preserves minutes in HHmm format", () => {
+    expect(getOnlineBoardDefaultTimeRange(true, new Date(2026, 4, 15, 14, 37))).toEqual({
+      timeFrom: "1337",
+      timeTo: "1737",
+    });
+  });
+});
diff --git a/src/features/online-board/timeDefaults.ts b/src/features/online-board/timeDefaults.ts
new file mode 100644
index 00000000..810b368a
--- /dev/null
+++ b/src/features/online-board/timeDefaults.ts
@@ -0,0 +1,38 @@
+/**
+ * Online-Board `Время рейса` default time range per TZ §4.1.1-R4 and R5.
+ *
+ * Desktop/tablet: always `00:00-24:00`.
+ * Mobile: [-1h, +3h] window centered on the user's current local time,
+ * clamped to [00:00, 24:00].
+ *
+ * Output uses `HHmm` strings (e.g. "0800" = 08:00). `2400` represents
+ * end-of-day inclusive, matching existing URL encoding.
+ */
+
+export interface TimeRange {
+  timeFrom: string;
+  timeTo: string;
+}
+
+export function getOnlineBoardDefaultTimeRange(
+  isMobile: boolean,
+  now: Date = new Date(),
+): TimeRange {
+  if (!isMobile) {
+    return { timeFrom: "0000", timeTo: "2400" };
+  }
+  const nowMinutes = now.getHours() * 60 + now.getMinutes();
+  const fromMinutes = Math.max(0, nowMinutes - 60);
+  const toMinutes = Math.min(24 * 60, nowMinutes + 3 * 60);
+  return {
+    timeFrom: minutesToHHmm(fromMinutes),
+    timeTo: minutesToHHmm(toMinutes),
+  };
+}
+
+function minutesToHHmm(total: number): string {
+  if (total === 24 * 60) return "2400";
+  const h = Math.floor(total / 60);
+  const m = total % 60;
+  return `${String(h).padStart(2, "0")}${String(m).padStart(2, "0")}`;
+}
diff --git a/src/shared/hooks/useIsMobileViewport.test.ts b/src/shared/hooks/useIsMobileViewport.test.ts
new file mode 100644
index 00000000..ac5c484b
--- /dev/null
+++ b/src/shared/hooks/useIsMobileViewport.test.ts
@@ -0,0 +1,54 @@
+/**
+ * @vitest-environment jsdom
+ */
+
+import { describe, expect, it, vi, beforeEach, afterEach } from "vitest";
+import { renderHook, act } from "@testing-library/react";
+import { useIsMobileViewport } from "./useIsMobileViewport.js";
+
+describe("useIsMobileViewport", () => {
+  let matchMediaMock: ReturnType<typeof vi.fn>;
+  let listeners: ((e: { matches: boolean }) => void)[];
+  let currentMatches = false;
+
+  beforeEach(() => {
+    listeners = [];
+    currentMatches = false;
+    matchMediaMock = vi.fn().mockImplementation((query: string) => ({
+      matches: currentMatches,
+      media: query,
+      addEventListener: (_type: string, listener: (e: { matches: boolean }) => void) => {
+        listeners.push(listener);
+      },
+      removeEventListener: () => {},
+      dispatchEvent: () => false,
+    }));
+    Object.defineProperty(window, "matchMedia", { value: matchMediaMock, configurable: true });
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("returns false when viewport is wider than mobile breakpoint", () => {
+    currentMatches = false;
+    const { result } = renderHook(() => useIsMobileViewport());
+    expect(result.current).toBe(false);
+  });
+
+  it("returns true when viewport is narrower than mobile breakpoint", () => {
+    currentMatches = true;
+    const { result } = renderHook(() => useIsMobileViewport());
+    expect(result.current).toBe(true);
+  });
+
+  it("updates when media-query match changes", () => {
+    currentMatches = false;
+    const { result } = renderHook(() => useIsMobileViewport());
+    expect(result.current).toBe(false);
+    act(() => {
+      for (const l of listeners) l({ matches: true });
+    });
+    expect(result.current).toBe(true);
+  });
+});
diff --git a/src/shared/hooks/useIsMobileViewport.ts b/src/shared/hooks/useIsMobileViewport.ts
new file mode 100644
index 00000000..fc1b02fa
--- /dev/null
+++ b/src/shared/hooks/useIsMobileViewport.ts
@@ -0,0 +1,29 @@
+import { useEffect, useState } from "react";
+
+/** Mobile breakpoint — matches the project's `$media-breakpoint-mobile` token (640px). */
+const MOBILE_BREAKPOINT_PX = 640;
+
+/**
+ * Returns `true` when the viewport is ≤ the mobile breakpoint. Used to
+ * branch TZ-prescribed mobile-only behaviors (e.g. Online-Board's
+ * `Время рейса` default = -1h/+3h per §4.1.1-R5).
+ *
+ * SSR: returns `false` initially, then updates on first client render.
+ */
+export function useIsMobileViewport(): boolean {
+  const [isMobile, setIsMobile] = useState<boolean>(() => {
+    if (typeof window === "undefined") return false;
+    return window.matchMedia(`(max-width: ${MOBILE_BREAKPOINT_PX}px)`).matches;
+  });
+
+  useEffect(() => {
+    if (typeof window === "undefined") return;
+    const mql = window.matchMedia(`(max-width: ${MOBILE_BREAKPOINT_PX}px)`);
+    const listener = (e: MediaQueryListEvent | { matches: boolean }) => setIsMobile(e.matches);
+    setIsMobile(mql.matches);
+    mql.addEventListener("change", listener as (e: MediaQueryListEvent) => void);
+    return () => mql.removeEventListener("change", listener as (e: MediaQueryListEvent) => void);
+  }, []);
+
+  return isMobile;
+}