diff --git a/DECISIONS.md b/DECISIONS.md
index d8451bc..1406aa3 100644
--- a/DECISIONS.md
+++ b/DECISIONS.md
@@ -766,3 +766,9 @@ Implementation decisions not spelled out in `memory-bank/implementation-plan.md`
 
 - **Field names:** `dutyLimitGameHours`, `longHaulDutyBonusGameHours`, `restGameHours`. All in game hours, like other trip times.
 - **The "rest ≥ board period" rule is a content test, not a schema refinement**, as the CR asks. The schema only requires the three numbers to be positive.
+
+## Step 10D.2
+
+- **Trip duty is measured from the dispatch's real departure and arrival times** (`tripGameHours`), so it is exactly the travel time the player saw, handling time included.
+- **"Contains a board boundary" is tested as "wakes in a later board period"**: `boardPeriodIndex(restEndsAt(start)) > boardPeriodIndex(start)`, which is what "wakes to a fresh board" means.
+- **Resting ends exactly at rest-ends-at** (`now < restEndsAt`), the same convention as dispatch arrival.
diff --git a/memory-bank/progress.md b/memory-bank/progress.md
index c044df0..fcd11f4 100644
--- a/memory-bank/progress.md
+++ b/memory-bank/progress.md
@@ -165,7 +165,7 @@ Spec: `memory-bank/change-requests/CR-003-map-art.md`. New decisions recorded as
 Spec: `memory-bank/change-requests/CR-004-pilot-duty-rest.md`. New decisions recorded as plan §0.5.42–0.5.46.
 
 - [x] **Step 10D.1 — Content config.** GameConfig gains `dutyLimitGameHours` (12), `longHaulDutyBonusGameHours` (2) and `restGameHours` (60 = one 60-minute board period at 60× compression). Tests: rest in real time ≥ board period; duty numbers positive.
-- [ ] Step 10D.2 — Sim: duty and rest functions
+- [x] **Step 10D.2 — Sim: duty and rest functions.** `packages/sim/src/duty.ts`: `dutyLimit(skills)`, `isResting(restEndsAt, now)`, `tripGameHours(departure, arrival)`, `dutyAfterTrip(duty, hours, tutorial)`, `needsRest(duty, limit)`, `restEndsAt(start)`. Test: `duty.test.ts` (including 2,000 random rest starts all waking in a later board period).
 - [ ] Step 10D.3 — Sim: validation and resolution
 - [ ] Step 10D.4 — Schema and database
 - [ ] Step 10D.5 — Server: apply duty and rest
diff --git a/packages/sim/src/duty.ts b/packages/sim/src/duty.ts
new file mode 100644
index 0000000..ff1beae
--- /dev/null
+++ b/packages/sim/src/duty.ts
@@ -0,0 +1,43 @@
+import type { GameConfig } from "@deadhead/content";
+import { MS_PER_HOUR, type RealTime } from "./clock";
+import { LONG_HAUL_SKILL_ID, skillLevel, type SkillLevels } from "./pilot";
+
+/**
+ * Pilot duty and rest (CR-004, decisions 0.5.42–0.5.46). Rest is derived from a
+ * stored rest-ends-at time, so nothing has to run when it ends.
+ */
+
+/** Game hours a pilot may fly before mandatory rest: base plus a Long Haul bonus. */
+export function dutyLimit(skills: SkillLevels, config: GameConfig): number {
+  const longHaul = skillLevel(skills, LONG_HAUL_SKILL_ID);
+  return config.dutyLimitGameHours + longHaul * config.longHaulDutyBonusGameHours;
+}
+
+/** Whether a pilot is resting at real time `now`: until (not at) rest-ends-at. */
+export function isResting(restEndsAt: RealTime | null, now: RealTime): boolean {
+  return restEndsAt !== null && now < restEndsAt;
+}
+
+/** Game hours of a trip from its real departure and arrival times. */
+export function tripGameHours(
+  departureAt: RealTime,
+  arrivalAt: RealTime,
+  config: GameConfig,
+): number {
+  return ((arrivalAt - departureAt) * config.timeCompression) / MS_PER_HOUR;
+}
+
+/** Duty hours after a trip: tutorial runs add nothing (decision 0.5.42). */
+export function dutyAfterTrip(dutyHours: number, tripHours: number, tutorial: boolean): number {
+  return tutorial ? dutyHours : dutyHours + tripHours;
+}
+
+/** Whether duty at or over the limit forces rest on arrival (decision 0.5.44). */
+export function needsRest(dutyHours: number, limit: number): boolean {
+  return dutyHours >= limit;
+}
+
+/** When a rest starting at real time `start` ends (decision 0.5.45). */
+export function restEndsAt(start: RealTime, config: GameConfig): RealTime {
+  return start + (config.restGameHours * MS_PER_HOUR) / config.timeCompression;
+}
diff --git a/packages/sim/src/index.ts b/packages/sim/src/index.ts
index f5c29b6..ab815e6 100644
--- a/packages/sim/src/index.ts
+++ b/packages/sim/src/index.ts
@@ -69,3 +69,5 @@ export type { LoanSplit } from "./loan";
 
 export { resolveDispatch } from "./resolution";
 export type { Resolution, ResolveInput, ResolveResult } from "./resolution";
+
+export { dutyAfterTrip, dutyLimit, isResting, needsRest, restEndsAt, tripGameHours } from "./duty";
diff --git a/packages/sim/test/duty.test.ts b/packages/sim/test/duty.test.ts
new file mode 100644
index 0000000..ba7ae09
--- /dev/null
+++ b/packages/sim/test/duty.test.ts
@@ -0,0 +1,57 @@
+import { content } from "@deadhead/content";
+import { describe, expect, it } from "vitest";
+import {
+  boardPeriodIndex,
+  createRng,
+  dutyAfterTrip,
+  dutyLimit,
+  isResting,
+  needsRest,
+  restEndsAt,
+  tripGameHours,
+} from "../src/index";
+
+const config = content.config;
+
+describe("pilot duty and rest (CR-004)", () => {
+  it("raises the duty limit by the bonus for each Long Haul level", () => {
+    expect(dutyLimit({}, config)).toBe(config.dutyLimitGameHours);
+    for (let level = 1; level <= 5; level++) {
+      expect(dutyLimit({ "long-haul": level }, config)).toBe(
+        config.dutyLimitGameHours + level * config.longHaulDutyBonusGameHours,
+      );
+    }
+    // Other skills do not count.
+    expect(dutyLimit({ "heavy-cargo": 3 }, config)).toBe(config.dutyLimitGameHours);
+  });
+
+  it("is resting until, but not at, rest-ends-at", () => {
+    const ends = Date.parse("2100-01-01T12:00:00Z");
+    expect(isResting(ends, ends - 1000)).toBe(true);
+    expect(isResting(ends, ends)).toBe(false);
+    expect(isResting(null, ends)).toBe(false);
+  });
+
+  it("adds a trip's hours to duty, except for tutorial runs", () => {
+    expect(dutyAfterTrip(3, 4, false)).toBe(7);
+    expect(dutyAfterTrip(3, 4, true)).toBe(3);
+    // One real minute is one game hour at 60× compression.
+    expect(tripGameHours(0, 4 * 60_000, config)).toBeCloseTo(4);
+  });
+
+  it("needs rest at or over the limit", () => {
+    expect(needsRest(dutyAfterTrip(10, 4, false), 12)).toBe(true);
+    expect(needsRest(dutyAfterTrip(4, 4, false), 12)).toBe(false);
+    expect(needsRest(12, 12)).toBe(true);
+  });
+
+  it("always wakes a rested pilot in a later board period", () => {
+    const rng = createRng(2026);
+    for (let i = 0; i < 2000; i++) {
+      const start = Math.floor(rng.next() * 1e13);
+      expect(boardPeriodIndex(restEndsAt(start, config), config)).toBeGreaterThan(
+        boardPeriodIndex(start, config),
+      );
+    }
+  });
+});