why not having
elseif attached unavailable_until as u then
    h.put_header_key_value ({HTTP_HEADER_NAMES}.header_retry_after, h.date_to_rfc1123_http_date_format (u))

this way ... no need to insult the compiler ;)
Note that we can add a 
HTTP_HEADER.put_retry_after (a_date: DATE_TIME)

this would look nicer.