Subtract term X from Y on the same log?

How can I Subtract term X from Y on the same log?
I saw this, but it doesn't do any arithmetical use. (This uses mutate).
I also tried through a scripted field in Kibana but the fields are not saved in ES and I need to later use them.

I also tried through the ruby filter:

    event['delay'] = (event['old1']-event['old2'])

but it didnt result with any thing

Help will be greatly appreciated.

Found an answer here:

filter {
  if [received_at] and [@timestamp] {
    ruby {
      init => "require 'time'"
      code => "
        received_by_indexer = Time.iso8601(event['received_at'].to_s).to_i;
        time_in_event = Time.iso8601(event['@timestamp'].to_s).to_i;
        event['time_difference_in_seconds'] = received_by_indexer - time_in_event;
        event['epoch_received_at_in_seconds'] = received_by_indexer;
        event['epoch_timestamp_in_seconds'] = time_in_event;
      add_tag => [ "calculated_time_difference" ]

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