You should already be able to implement this using the If-Modified-Since HTTP header. The server only needs to send you back articles in the RSS feed that have

You should already be able to implement this using the If-Modified-Since HTTP header. The server only needs to send you back articles in the RSS feed that have been added after that date. The If-Modified-Since value is meant to come from the previous responses Last-Modified (not a timestamp you make up), so that side-steps timezone issues.