test.rb

timestamp_conversion = proc do |k,v|
  new_k = k == "timestampMs" ? Time.at(k[1..-4]) : k
  new_v = case v
  when Array
    v.map do |e|
      case e
      when Hash
        e.map(&:timestamp_conversion)
      else
        e
      end
    end
  when Hash
    v.map(&:timestamp_conversion)
  else
    v
  end

  [new_k,new_v]
end