diff --git a/src/shared/dateWindow.test.ts b/src/shared/dateWindow.test.ts
new file mode 100644
index 00000000..ca785be6
--- /dev/null
+++ b/src/shared/dateWindow.test.ts
@@ -0,0 +1,100 @@
+import { describe, expect, it, vi, beforeEach, afterEach } from "vitest";
+import {
+  BOARD_WINDOW_DAYS_BACK,
+  BOARD_WINDOW_DAYS_FORWARD,
+  SCHEDULE_WINDOW_DAYS_BACK,
+  SCHEDULE_WINDOW_DAYS_FORWARD,
+  MAP_WINDOW_DAYS_BACK,
+  MAP_WINDOW_MONTHS_FORWARD,
+  isInBoardWindow,
+  isInScheduleWindow,
+  isInMapWindow,
+  boardWindowBounds,
+  scheduleWindowBounds,
+  mapWindowBounds,
+} from "./dateWindow.js";
+
+describe("dateWindow constants", () => {
+  it("exports the TZ-defined numerical bounds", () => {
+    expect(BOARD_WINDOW_DAYS_BACK).toBe(1);
+    expect(BOARD_WINDOW_DAYS_FORWARD).toBe(14);
+    expect(SCHEDULE_WINDOW_DAYS_BACK).toBe(1);
+    expect(SCHEDULE_WINDOW_DAYS_FORWARD).toBe(330);
+    expect(MAP_WINDOW_DAYS_BACK).toBe(1);
+    expect(MAP_WINDOW_MONTHS_FORWARD).toBe(6);
+  });
+});
+
+describe("dateWindow helpers (clock frozen at 2026-05-15)", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date(2026, 4, 15, 12, 0, 0)); // May 15, 2026 noon local time
+  });
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  // Helper to format date as YYYY-MM-DD for comparison
+  function toYmd(d: Date): string {
+    return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, "0")}-${String(d.getDate()).padStart(2, "0")}`;
+  }
+
+  describe("4.1.2-R12: isInBoardWindow (-1/+14 days)", () => {
+    it("accepts today (yyyyMMdd)", () => {
+      expect(isInBoardWindow("20260515")).toBe(true);
+    });
+    it("accepts yesterday (-1 day)", () => {
+      expect(isInBoardWindow("20260514")).toBe(true);
+    });
+    it("rejects -2 days", () => {
+      expect(isInBoardWindow("20260513")).toBe(false);
+    });
+    it("accepts today + 14 days", () => {
+      expect(isInBoardWindow("20260529")).toBe(true);
+    });
+    it("rejects today + 15 days", () => {
+      expect(isInBoardWindow("20260530")).toBe(false);
+    });
+    it("rejects malformed input", () => {
+      expect(isInBoardWindow("")).toBe(false);
+      expect(isInBoardWindow("2026-05-15")).toBe(false);
+      expect(isInBoardWindow("20269999")).toBe(false);
+    });
+  });
+
+  describe("4.1.2-R12: isInScheduleWindow (-1/+330 days)", () => {
+    it("accepts today + 330 days", () => {
+      expect(isInScheduleWindow("20270410")).toBe(true);
+    });
+    it("rejects today + 331 days", () => {
+      expect(isInScheduleWindow("20270411")).toBe(false);
+    });
+    it("rejects -2 days", () => {
+      expect(isInScheduleWindow("20260513")).toBe(false);
+    });
+  });
+
+  describe("4.1.2-R12: isInMapWindow (-1 day / +6 months)", () => {
+    it("accepts today + 6 months (same calendar day)", () => {
+      expect(isInMapWindow("20261115")).toBe(true);
+    });
+    it("rejects today + 6 months + 1 day", () => {
+      expect(isInMapWindow("20261116")).toBe(false);
+    });
+    it("rejects -2 days", () => {
+      expect(isInMapWindow("20260513")).toBe(false);
+    });
+  });
+
+  it("exposes bounds tuples for UI consumers", () => {
+    const [bMin, bMax] = boardWindowBounds();
+    expect(toYmd(bMin)).toBe("2026-05-14");
+    expect(toYmd(bMax)).toBe("2026-05-29");
+    const [sMin, sMax] = scheduleWindowBounds();
+    expect(toYmd(sMin)).toBe("2026-05-14");
+    expect(toYmd(sMax)).toBe("2027-04-10");
+    const [mMin, mMax] = mapWindowBounds();
+    expect(toYmd(mMin)).toBe("2026-05-14");
+    expect(toYmd(mMax)).toBe("2026-11-15");
+  });
+});
diff --git a/src/shared/dateWindow.ts b/src/shared/dateWindow.ts
new file mode 100644
index 00000000..14d7b9aa
--- /dev/null
+++ b/src/shared/dateWindow.ts
@@ -0,0 +1,82 @@
+/**
+ * Centralized date-window constants and membership helpers per TZ §4.1.2.
+ *
+ * All date arguments are "yyyyMMdd" strings (TZ URL date format).
+ * Bounds are inclusive on both ends. Day-level comparison (time zeroed).
+ */
+
+export const BOARD_WINDOW_DAYS_BACK = 1;
+export const BOARD_WINDOW_DAYS_FORWARD = 14;
+export const SCHEDULE_WINDOW_DAYS_BACK = 1;
+export const SCHEDULE_WINDOW_DAYS_FORWARD = 330;
+export const MAP_WINDOW_DAYS_BACK = 1;
+export const MAP_WINDOW_MONTHS_FORWARD = 6;
+
+function today(): Date {
+  const d = new Date();
+  d.setHours(0, 0, 0, 0);
+  return d;
+}
+
+function addDays(base: Date, days: number): Date {
+  const d = new Date(base);
+  d.setDate(d.getDate() + days);
+  return d;
+}
+
+function addMonths(base: Date, months: number): Date {
+  const d = new Date(base);
+  d.setMonth(d.getMonth() + months);
+  return d;
+}
+
+function parseYyyymmdd(s: string): Date | null {
+  if (!/^\d{8}$/.test(s)) return null;
+  const y = Number(s.slice(0, 4));
+  const m = Number(s.slice(4, 6));
+  const d = Number(s.slice(6, 8));
+  if (m < 1 || m > 12 || d < 1 || d > 31) return null;
+  const dt = new Date(y, m - 1, d);
+  if (dt.getFullYear() !== y || dt.getMonth() !== m - 1 || dt.getDate() !== d) return null;
+  return dt;
+}
+
+function inRange(date: Date, min: Date, max: Date): boolean {
+  return date.getTime() >= min.getTime() && date.getTime() <= max.getTime();
+}
+
+export function boardWindowBounds(): [Date, Date] {
+  const base = today();
+  return [addDays(base, -BOARD_WINDOW_DAYS_BACK), addDays(base, BOARD_WINDOW_DAYS_FORWARD)];
+}
+
+export function scheduleWindowBounds(): [Date, Date] {
+  const base = today();
+  return [addDays(base, -SCHEDULE_WINDOW_DAYS_BACK), addDays(base, SCHEDULE_WINDOW_DAYS_FORWARD)];
+}
+
+export function mapWindowBounds(): [Date, Date] {
+  const base = today();
+  return [addDays(base, -MAP_WINDOW_DAYS_BACK), addMonths(base, MAP_WINDOW_MONTHS_FORWARD)];
+}
+
+export function isInBoardWindow(yyyymmdd: string): boolean {
+  const d = parseYyyymmdd(yyyymmdd);
+  if (!d) return false;
+  const [min, max] = boardWindowBounds();
+  return inRange(d, min, max);
+}
+
+export function isInScheduleWindow(yyyymmdd: string): boolean {
+  const d = parseYyyymmdd(yyyymmdd);
+  if (!d) return false;
+  const [min, max] = scheduleWindowBounds();
+  return inRange(d, min, max);
+}
+
+export function isInMapWindow(yyyymmdd: string): boolean {
+  const d = parseYyyymmdd(yyyymmdd);
+  if (!d) return false;
+  const [min, max] = mapWindowBounds();
+  return inRange(d, min, max);
+}