---
description: Dynamically set a cookie expiration and test group.
title: Append dates to cookies to use with A/B testing
image: https://developers.cloudflare.com/og-docs.png
---

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# Append dates to cookies to use with A/B testing

Dynamically set a cookie expiration and test group.

Last updated Aug 24, 2026|Copy as Markdown|[View as Markdown](https://docs-durable-objects-instance-replaced-errors.previews.developers.cloudflare.com/rules/snippets/examples/append-dates-to-cookies/index.md)|[Agent setup](https://docs-durable-objects-instance-replaced-errors.previews.developers.cloudflare.com/agent-setup/)

```js
export default {
	async fetch(request) {
		const response = await fetch(request);

		// Clone the response so that it is no longer immutable
		const newResponse = new Response(response.body, response);

		// Define the dynamic expiry time. 24 h * 60 m * 60 s * 1000 ms = 86,400,000 ms
		const expiry = new Date(Date.now() + 7 * 86400000).toUTCString();
		// Define the group variable. "A" if the request header "userGroup" is "premium", "B" if otherwise.
		const group = request.headers.get("userGroup") == "premium" ? "A" : "B";

		// Append the custom header with the values
		newResponse.headers.append(
			"Set-Cookie",
			`testGroup=${group}; Expires=${expiry}; Path=/`,
		);

		return newResponse;
	},
};
```

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```json
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```