(reap.decoders/http-date "Thu, 31 Dec 1998 23:59:60 GMT") should work, as specified in RFC7231

     time-of-day  = hour ":" minute ":" second
                  ; 00:00:00 - 23:59:60 (leap second)

Java time does not support leap seconds. The DateTimeFormatter for "appendInstant" instead will round down to :59 and set a flag to indicate the leap second was true and allow you to act on that.