diff --git a/nip40.test.ts b/nip40.test.ts
new file mode 100644
index 0000000..f484329
--- /dev/null
+++ b/nip40.test.ts
@@ -0,0 +1,44 @@
+import { describe, test, expect, jest } from 'bun:test'
+import { buildEvent } from './test-helpers.ts'
+import { getExpiration, isEventExpired, waitForExpire, onExpire } from './nip40.ts'
+
+describe('getExpiration', () => {
+  test('returns the expiration as a Date object', () => {
+    const event = buildEvent({ tags: [['expiration', '123']] })
+    const result = getExpiration(event)
+    expect(result).toEqual(new Date(123000))
+  })
+})
+
+describe('isEventExpired', () => {
+  test('returns true when the event has expired', () => {
+    const event = buildEvent({ tags: [['expiration', '123']] })
+    const result = isEventExpired(event)
+    expect(result).toEqual(true)
+  })
+
+  test('returns false when the event has not expired', () => {
+    const future = Math.floor(Date.now() / 1000) + 10
+    const event = buildEvent({ tags: [['expiration', future.toString()]] })
+    const result = isEventExpired(event)
+    expect(result).toEqual(false)
+  })
+})
+
+describe('waitForExpire', () => {
+  test('returns a promise that resolves when the event expires', async () => {
+    const event = buildEvent({ tags: [['expiration', '123']] })
+    const result = await waitForExpire(event)
+    expect(result).toEqual(event)
+  })
+})
+
+describe('onExpire', () => {
+  test('calls the callback when the event expires', async () => {
+    const event = buildEvent({ tags: [['expiration', '123']] })
+    const callback = jest.fn()
+    onExpire(event, callback)
+    await new Promise((resolve) => setTimeout(resolve, 200))
+    expect(callback).toHaveBeenCalled()
+  })
+})
diff --git a/nip40.ts b/nip40.ts
new file mode 100644
index 0000000..3ce45fe
--- /dev/null
+++ b/nip40.ts
@@ -0,0 +1,49 @@
+import { Event } from './core.ts'
+
+/** Get the expiration of the event as a `Date` object, if any. */
+function getExpiration(event: Event): Date | undefined {
+  const tag = event.tags.find(([name]) => name === 'expiration')
+  if (tag) {
+    return new Date(parseInt(tag[1]) * 1000)
+  }
+}
+
+/** Check if the event has expired. */
+function isEventExpired(event: Event): boolean {
+  const expiration = getExpiration(event)
+  if (expiration) {
+    return Date.now() > expiration.getTime()
+  } else {
+    return false
+  }
+}
+
+/** Returns a promise that resolves when the event expires. */
+async function waitForExpire(event: Event): Promise<Event> {
+  const expiration = getExpiration(event)
+  if (expiration) {
+    const diff = expiration.getTime() - Date.now()
+    if (diff > 0) {
+      await sleep(diff)
+      return event
+    } else {
+      return event
+    }
+  } else {
+    throw new Error('Event has no expiration')
+  }
+}
+
+/** Calls the callback when the event expires. */
+function onExpire(event: Event, callback: (event: Event) => void): void {
+  waitForExpire(event)
+    .then(callback)
+    .catch(() => {})
+}
+
+/** Resolves when the given number of milliseconds have elapsed. */
+function sleep(ms: number): Promise<void> {
+  return new Promise((resolve) => setTimeout(resolve, ms))
+}
+
+export { getExpiration, isEventExpired, waitForExpire, onExpire }