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

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

Dynamically set a cookie expiration and test group.

JavaScript

```
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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```