Overwriting json @timestamp

I added this now, but I still get the same error message:

input {
beats {
port => 5445
codec => "json"
ssl => true
ssl_certificate => "/etc/logstash/logstash.crt"
ssl_key => "/etc/logstash/logstash.key"
}
}

filter {
json {
source => "message"
}
date {
match => [ "timestamp", "ISO8601" ]
}
}