setCookie example in docs incorrectly handles expires and maxAge

[will-stone]

The example code for a setCookie function (https://nextjs.org/docs/api-routes/api-middlewares#extending-the-reqres-objects-with-typescript) shows the maxAge option being divided by 1000.

I believe the function should be:

if (typeof options.maxAge === 'number') {
  options.expires = new Date(Date.now() + options.maxAge * 1000);
}

The max-age attribute for Set-Cookie is in seconds. Therefore I believe there's a few changes that are required to the middle conditional:

1. The check should only be concerned with whether the maxAge is numerical.
2. As Date.now provides time in milliseconds, we need to multiply options.maxAge by 1000.
3. options.maxAge does not require altering as it can be passed on in seconds to the serialize function which expects it in seconds:

• https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie#attributes
• https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/cookie/index.d.ts#L55

Originally posted by @will-stone in #27617 (comment)

[balazsorban44]

This is a good point in my opinion, that ties in with #36478 (comment)

[ghost]

Hi, this is my first open source contribution. I have raised a pull request for this issue:#36870 . Please review my changes and let my know any changes that I ought to do. Thanks

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