In addition to starting at the latest feed, you can pass a
query string parameter to get the newest post before that date. This date
must be in the ISO 8601 UTC format, but can be partial, e.g.
2020-06 are all valid.
This is useful for navigating archives by date, without having to start at the beginning and walk backwards.
before date is earlier than the earliest post, the
server returns a 404 Not Found response.