How the Date Header Affects Cookie Expiration and Caching

tl;dr

When a cookie includes an Expires attribute or an HTTP response includes caching headers like Expires or Cache-Control, their validity depends on the server's Date header if present. Otherwise, the browser uses its local time. This can lead to issues in tests with mocked time or inconsistent cache behavior.

Cookie Expires depends on the Date header or browser time

When a cookie includes an Expires attribute, the browser evaluates the expiration date relative to a reference time:

1. If the HTTP response includes a Date header, the browser uses this time as the reference.
2. If no Date header is present, the browser falls back to its own local system time.

This subtle behavior ensures that cookies remain consistent with server time, even if the client’s clock is incorrect. However, this can lead to unexpected issues in specific scenarios.

Why does this matter?

Mocked time in tests

Consider a Rails application where you use the remember_me feature of Clearance for authentication. The cookie for this feature might include an Expires attribute, specifying its validity:

CopySet-Cookie: remember_me=abc123; Expires=Fri, 15 Dec 2023 12:00:00 GMT

When running tests, you might mock the time (e.g., using Timecop) to simulate past or future dates. If the HTTP response also includes a Date header, the browser uses this as the reference for the Expires calculation:

CopyDate: Fri, 08 Dec 2023 12:00:00 GMT

In this case, the cookie is evaluated as valid until Fri, 15 Dec 2023 12:00:00 GMT, regardless of the browser's local time.

However, if the Date header is missing, the browser relies on its own local clock. If your mocked time is in the past relative to the expiration, the cookie might incorrectly appear as expired.

Relevance for Caching

This behavior is also relevant for HTTP caching. Headers like Expires or Cache-Control: max-age use the Date header as a reference time. If the Date header is missing, the browser relies on its local time, which can lead to inconsistent cache behavior. Ensure your responses include a Date header to avoid issues with cache expiration.

How to handle this in tests?

Ensure your server sets a consistent Date header, especially in test environments. If your test mocks time and you observe premature cookie expiration, consider adding middleware to enforce a Date header:

Copyclass AddDateHeaderMiddleware
  def initialize(app)
    @app = app
  end

  def call(env)
    status, headers, body = @app.call(env)

    if headers['Date'].blank?
      headers['Date'] = Time.now.httpdate
    end

    [status, headers, body]
  end
end