PSA: Fix handling invalid date "0" for Expires header value as expired

[Ryo Ogawa]

Hello all,

We fixed the handling of the invalid Expires header value “0” according to RFC 9111. Previously, Expires: 0 was just ignored, but now it is treated as an expired value.

In almost all cases no action from web developers is required. However, if you see an unwanted increase in 304 Not Modified responses in your server logs or unwanted cache miss, you may wish to check the Expires: header that your server is sending using DevTools.

Here are the related references:

• RFC: https://httpwg.org/specs/rfc9111.html#rfc.section.5.3
• Gerrit: https://crrev.com/c/4021895
• crbug: https://crbug.com/853508

Thanks!