diff --git a/src/timestamps.test.ts b/src/timestamps.test.ts
new file mode 100644
index 0000000..fa344b9
--- /dev/null
+++ b/src/timestamps.test.ts
@@ -0,0 +1,109 @@
+import { createServer } from 'node:http'
+import { gunzipSync } from 'node:zlib'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { PostHogPlugin } from './index.js'
+
+type AnyEvent = { type: string; properties: Record<string, unknown> }
+
+const originalEnv = {
+    apiKey: process.env.POSTHOG_API_KEY,
+    enabled: process.env.POSTHOG_ENABLED,
+    host: process.env.POSTHOG_HOST,
+    timezone: process.env.TZ,
+}
+
+afterEach(() => {
+    vi.useRealTimers()
+    for (const [key, value] of Object.entries({
+        POSTHOG_API_KEY: originalEnv.apiKey,
+        POSTHOG_ENABLED: originalEnv.enabled,
+        POSTHOG_HOST: originalEnv.host,
+        TZ: originalEnv.timezone,
+    })) {
+        if (value === undefined) delete process.env[key]
+        else process.env[key] = value
+    }
+})
+
+describe('event timestamps', () => {
+    it('lets posthog-node add an exact UTC timestamp to the serialized event', async () => {
+        process.env.TZ = 'America/Los_Angeles'
+        const localTime = new Date(2024, 6, 1, 12, 34, 56, 789)
+        expect(localTime.getTimezoneOffset()).toBe(420)
+        vi.useFakeTimers({ toFake: ['Date'] })
+        vi.setSystemTime(localTime)
+
+        const bodies: Buffer[] = []
+        const server = createServer((request, response) => {
+            const chunks: Buffer[] = []
+            request.on('data', (chunk: Buffer) => chunks.push(chunk))
+            request.on('end', () => {
+                const body = Buffer.concat(chunks)
+                bodies.push(request.headers['content-encoding'] === 'gzip' ? gunzipSync(body) : body)
+                response.writeHead(200, { 'content-type': 'application/json' })
+                response.end('{}')
+            })
+        })
+        await new Promise<void>((resolve) => server.listen(0, '127.0.0.1', resolve))
+
+        try {
+            const address = server.address()
+            if (!address || typeof address === 'string') throw new Error('Expected TCP server address')
+            process.env.POSTHOG_API_KEY = 'phc_test'
+            process.env.POSTHOG_ENABLED = 'true'
+            process.env.POSTHOG_HOST = `http://127.0.0.1:${address.port}`
+
+            const hooks = (await PostHogPlugin({} as never)) as {
+                event: (input: { event: AnyEvent }) => Promise<void>
+                dispose: () => Promise<void>
+            }
+            const sessionID = 'session-timestamp'
+            const messageID = 'message-timestamp'
+
+            await hooks.event({
+                event: {
+                    type: 'message.updated',
+                    properties: {
+                        info: {
+                            role: 'assistant',
+                            id: messageID,
+                            sessionID,
+                            modelID: 'test-model',
+                            providerID: 'test-provider',
+                        },
+                    },
+                },
+            })
+            await hooks.event({
+                event: {
+                    type: 'message.part.updated',
+                    properties: {
+                        part: {
+                            type: 'step-finish',
+                            sessionID,
+                            messageID,
+                            tokens: { input: 1, output: 1, reasoning: 0, cache: { read: 0, write: 0 } },
+                            cost: 0,
+                            reason: 'stop',
+                        },
+                    },
+                },
+            })
+            await hooks.dispose()
+
+            expect(bodies).toHaveLength(1)
+            const payload = JSON.parse(bodies[0].toString()) as {
+                sent_at: string
+                batch: Array<{ event: string; timestamp: string }>
+            }
+            expect(payload.sent_at).toBe('2024-07-01T19:34:56.789Z')
+            expect(payload.batch).toHaveLength(1)
+            expect(payload.batch[0]).toMatchObject({
+                event: '$ai_generation',
+                timestamp: '2024-07-01T19:34:56.789Z',
+            })
+        } finally {
+            await new Promise<void>((resolve, reject) => server.close((error) => (error ? reject(error) : resolve())))
+        }
+    })
+})