Server should include the Last-Modified header

[csarven]

Server MUST generate a Last-Modified header field in response to GET and HEAD requests.

https://solidproject.org/ED/protocol#server-last-modified-includes

Servers are strongly discouraged from exposing information beyond the minimum amount necessary to enable a feature.

https://solidproject.org/ED/protocol#exposing-information

So, the implementation should only expose the Last-Modified header when access is granted to the request target, i.e., when user or public has Read access to the resource. This is typically aligned with the 200 response.

[csarven]

The modified timestamp can be easily retrieved using fs.stat and mtime.

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Here is a screencast showing an application making use of the header field in one particular way:

https://dokie.li/media/video/dokieli-last-modified.webm

dokieli-last-modified.webm

See also solid/specification#678 (comment)