An RSS feed is carried over HTTP. As such, it is expected that the client use the HTTP "if-modified-since" request header to indicate what it wants. Usually the time/date in this field is the same as the time/date in the "last-modified" response header the last time the same resource (file, or in your case, RSS feed) was received via HTTP.

An RSS feed can be as simple as a file, for example, "feed.rss", updated as needed. In that case, the file can be served by an ordinary webserver and the value of the "last-modified" response header will be the modified time of the file.

Or, an RSS feed can be generated on demand. In that case, the "last-modified" response header will often be the time of generattion. It could also be the time of the most recent item the feed generator is tracking.

Either way, the RSS client can specify what it last "saw" using the "if-modified-since" request header. The webserver or feed generator can then act accordingly.