diff --git a/test/time/boundary-examples.test.ts b/test/time/boundary-examples.test.ts
new file mode 100644
index 0000000..64c3b41
--- /dev/null
+++ b/test/time/boundary-examples.test.ts
@@ -0,0 +1,43 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { useFakeClock } from './fakeClock'
+import { minutesAgo } from './offsets'
+
+afterEach(() => vi.useRealTimers())
+
+// JS world: mirrors the debounce comparison at
+// src/features/webhook/dropbox/lib/webhook.service.ts:42, reproduced purely
+// (no DB, no prod import — DEBOUNCE_WINDOW_MS is module-local in prod).
+describe('JS-world boundary example: webhook debounce', () => {
+  const DEBOUNCE_WINDOW_MS = 5 * 60 * 1000 // documentation copy of the prod window
+
+  const isRecentlySynced = (lastStartedAt: Date) =>
+    lastStartedAt >= new Date(Date.now() - DEBOUNCE_WINDOW_MS)
+
+  it('a stamp 4 minutes ago is inside the window (debounced → defer to cron)', () => {
+    const clock = useFakeClock()
+    expect(isRecentlySynced(minutesAgo(4))).toBe(true)
+    clock.restore()
+  })
+
+  it('a stamp 6 minutes ago is outside the window (sync triggers)', () => {
+    const clock = useFakeClock()
+    expect(isRecentlySynced(minutesAgo(6))).toBe(false)
+    clock.restore()
+  })
+})
+
+// SQL world: the resync backoff compares in Postgres (NOW()), which JS cannot
+// fake. This documents the seed-offset shape; the real assertion against real
+// Postgres lands in L2.3, not here.
+describe('SQL-world boundary template: resync backoff', () => {
+  it('documents the past-backoff seed offset (real assertion in L2.3)', () => {
+    // Integration usage:
+    //   await fileSyncSeeder.create(
+    //     pendingCreate(PendingActionTarget.DROPBOX),
+    //     { pendingActionLastAttemptAt: minutesAgo(6) }, // past 5min × 1 attempt
+    //   )
+    //   // then assert findFailedSyncs() (SQL NOW()) INCLUDES the row.
+    const seededAt = minutesAgo(6)
+    expect(seededAt.getTime()).toBeLessThan(Date.now())
+  })
+})
diff --git a/test/time/fakeClock.test.ts b/test/time/fakeClock.test.ts
new file mode 100644
index 0000000..0de28a0
--- /dev/null
+++ b/test/time/fakeClock.test.ts
@@ -0,0 +1,53 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { sleep } from '@/utils/sleep'
+import { useFakeClock } from './fakeClock'
+
+afterEach(() => vi.useRealTimers())
+
+describe('useFakeClock', () => {
+  it('freezes now at the given instant', () => {
+    const clock = useFakeClock('2026-03-01T12:00:00.000Z')
+    expect(new Date().toISOString()).toBe('2026-03-01T12:00:00.000Z')
+    expect(Date.now()).toBe(new Date('2026-03-01T12:00:00.000Z').getTime())
+    clock.restore()
+  })
+
+  it('advances deterministically with advanceByMinutes', async () => {
+    const clock = useFakeClock('2026-03-01T12:00:00.000Z')
+    await clock.advanceByMinutes(5)
+    expect(new Date().toISOString()).toBe('2026-03-01T12:05:00.000Z')
+    clock.restore()
+  })
+
+  it('advances hours', async () => {
+    const clock = useFakeClock('2026-03-01T12:00:00.000Z')
+    await clock.advanceByHours(2)
+    expect(new Date().toISOString()).toBe('2026-03-01T14:00:00.000Z')
+    clock.restore()
+  })
+
+  it('flushes a real sleep() without really waiting', async () => {
+    const clock = useFakeClock()
+    let resolved = false
+    const p = sleep(5000).then(() => {
+      resolved = true
+    })
+    await clock.advanceBy(5000)
+    await p
+    expect(resolved).toBe(true)
+    clock.restore()
+  })
+
+  it('setNow jumps to an absolute instant', () => {
+    const clock = useFakeClock()
+    clock.setNow(new Date('2030-01-01T00:00:00.000Z'))
+    expect(new Date().getFullYear()).toBe(2030)
+    clock.restore()
+  })
+
+  it('restore returns to real time', () => {
+    const clock = useFakeClock('2000-01-01T00:00:00.000Z')
+    clock.restore()
+    expect(new Date().getFullYear()).toBeGreaterThan(2020)
+  })
+})
diff --git a/test/time/fakeClock.ts b/test/time/fakeClock.ts
new file mode 100644
index 0000000..c617457
--- /dev/null
+++ b/test/time/fakeClock.ts
@@ -0,0 +1,38 @@
+import { vi } from 'vitest'
+
+const MINUTE_MS = 60_000
+const HOUR_MS = 3_600_000
+
+// Faked sources exclude nextTick/queueMicrotask so promises still resolve.
+const FAKED = ['Date', 'setTimeout', 'clearTimeout', 'setInterval', 'clearInterval'] as const
+
+export interface FakeClock {
+  advanceBy(ms: number): Promise<void>
+  advanceByMinutes(n: number): Promise<void>
+  advanceByHours(n: number): Promise<void>
+  setNow(date: Date): void
+  restore(): void
+}
+
+// Freezes time for one test. Advancing is async so pending sleep() calls flush.
+export function useFakeClock(iso = '2026-01-01T00:00:00.000Z'): FakeClock {
+  vi.useFakeTimers({ toFake: [...FAKED] })
+  vi.setSystemTime(new Date(iso))
+  return {
+    async advanceBy(ms) {
+      await vi.advanceTimersByTimeAsync(ms)
+    },
+    async advanceByMinutes(n) {
+      await vi.advanceTimersByTimeAsync(n * MINUTE_MS)
+    },
+    async advanceByHours(n) {
+      await vi.advanceTimersByTimeAsync(n * HOUR_MS)
+    },
+    setNow(date) {
+      vi.setSystemTime(date)
+    },
+    restore() {
+      vi.useRealTimers()
+    },
+  }
+}
diff --git a/test/time/index.ts b/test/time/index.ts
new file mode 100644
index 0000000..d9a3be1
--- /dev/null
+++ b/test/time/index.ts
@@ -0,0 +1,3 @@
+export * from './fakeClock'
+export * from './offsets'
+export * from './sleep'
diff --git a/test/time/offsets.test.ts b/test/time/offsets.test.ts
new file mode 100644
index 0000000..9fa31d5
--- /dev/null
+++ b/test/time/offsets.test.ts
@@ -0,0 +1,28 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { useFakeClock } from './fakeClock'
+import { daysAgo, fromNow, hoursAgo, minutesAgo, msAgo, secondsAgo } from './offsets'
+
+afterEach(() => vi.useRealTimers())
+
+describe('offset helpers (real time)', () => {
+  it('minutesAgo returns a time before now', () => {
+    expect(minutesAgo(5).getTime()).toBeLessThan(Date.now())
+  })
+
+  it('fromNow returns a time after now', () => {
+    expect(fromNow(10_000).getTime()).toBeGreaterThan(Date.now())
+  })
+})
+
+describe('offset helpers (frozen clock — exact)', () => {
+  it('are exact relative to frozen now', () => {
+    const clock = useFakeClock('2026-06-01T00:00:00.000Z')
+    expect(msAgo(500).toISOString()).toBe('2026-05-31T23:59:59.500Z')
+    expect(secondsAgo(30).toISOString()).toBe('2026-05-31T23:59:30.000Z')
+    expect(minutesAgo(5).toISOString()).toBe('2026-05-31T23:55:00.000Z')
+    expect(hoursAgo(2).toISOString()).toBe('2026-05-31T22:00:00.000Z')
+    expect(daysAgo(1).toISOString()).toBe('2026-05-31T00:00:00.000Z')
+    expect(fromNow(60_000).toISOString()).toBe('2026-06-01T00:01:00.000Z')
+    clock.restore()
+  })
+})
diff --git a/test/time/offsets.ts b/test/time/offsets.ts
new file mode 100644
index 0000000..d410320
--- /dev/null
+++ b/test/time/offsets.ts
@@ -0,0 +1,30 @@
+// Date-math off the live Date.now(). Freeze with useFakeClock() for exact
+// values; against real Postgres it tracks real NOW() for SQL-side tests.
+const SECOND_MS = 1_000
+const MINUTE_MS = 60_000
+const HOUR_MS = 3_600_000
+const DAY_MS = 86_400_000
+
+export function msAgo(n: number): Date {
+  return new Date(Date.now() - n)
+}
+
+export function secondsAgo(n: number): Date {
+  return new Date(Date.now() - n * SECOND_MS)
+}
+
+export function minutesAgo(n: number): Date {
+  return new Date(Date.now() - n * MINUTE_MS)
+}
+
+export function hoursAgo(n: number): Date {
+  return new Date(Date.now() - n * HOUR_MS)
+}
+
+export function daysAgo(n: number): Date {
+  return new Date(Date.now() - n * DAY_MS)
+}
+
+export function fromNow(ms: number): Date {
+  return new Date(Date.now() + ms)
+}
diff --git a/test/time/sleep.test.ts b/test/time/sleep.test.ts
new file mode 100644
index 0000000..3238abc
--- /dev/null
+++ b/test/time/sleep.test.ts
@@ -0,0 +1,22 @@
+import { describe, expect, it, vi } from 'vitest'
+import { sleep } from '@/utils/sleep'
+import { mockSleepInstant } from './sleep'
+
+// Hoisted: auto-mocks @/utils/sleep for this whole file.
+vi.mock('@/utils/sleep')
+
+describe('mockSleepInstant', () => {
+  it('resolves immediately and records the requested duration', async () => {
+    const spy = mockSleepInstant()
+    await sleep(5000)
+    expect(spy).toHaveBeenCalledWith(5000)
+  })
+
+  it('records multiple calls in order without waiting', async () => {
+    const spy = mockSleepInstant()
+    await sleep(800)
+    await sleep(5000)
+    expect(spy).toHaveBeenNthCalledWith(1, 800)
+    expect(spy).toHaveBeenNthCalledWith(2, 5000)
+  })
+})
diff --git a/test/time/sleep.ts b/test/time/sleep.ts
new file mode 100644
index 0000000..44f821c
--- /dev/null
+++ b/test/time/sleep.ts
@@ -0,0 +1,12 @@
+import { type Mock, vi } from 'vitest'
+import { sleep } from '@/utils/sleep'
+
+// Instant spy for sleep() — assert the wait (e.g. toHaveBeenCalledWith(5000))
+// with no real delay. Caller's file must hoist vi.mock('@/utils/sleep') first.
+export function mockSleepInstant(): Mock<typeof sleep> {
+  const spy = vi.mocked(sleep)
+  // Reset call history — clearMocks is off in this project.
+  spy.mockClear()
+  spy.mockImplementation(() => Promise.resolve())
+  return spy
+}
diff --git a/vitest.setup.ts b/vitest.setup.ts
index d9f0a5f..1db7007 100644
--- a/vitest.setup.ts
+++ b/vitest.setup.ts
@@ -1,3 +1,4 @@
+import { afterEach, vi } from 'vitest'
 import { applyPlaceholderServerEnv } from './test/support/placeholder-env'
 
 // Provide placeholder env vars so server-only modules that validate via Zod
@@ -5,3 +6,10 @@ import { applyPlaceholderServerEnv } from './test/support/placeholder-env'
 // values for behavioral assertions — they exist solely to satisfy schema parse.
 applyPlaceholderServerEnv()
 process.env.DATABASE_URL ??= 'postgresql://test:test@localhost:5432/test'
+
+// Safety net: if a test froze time via useFakeClock() and forgot to restore(),
+// reset to real timers so it can't leak into the next test. A no-op when timers
+// were never faked.
+afterEach(() => {
+  vi.useRealTimers()
+})