time = Fluent::EventTime.now
    # previous version
    p time # => #<Fluent::EventTime:0x00005635d50ecdb8 @sec=1522311320, @nsec=861617325>
    # this version
    p time # => 2018-03-29 17:15:20.861617325 +0900

Because 3 arguments version of `Time.at` has been added since Ruby 2.5.0.
See also https://bugs.ruby-lang.org/issues/13919

Strftime is faster than Time#strftime.

```
$ bundle exec ruby t.rb
Rehearsal -------------------------------------------------
Time#strftime   4.380378   0.835296   5.215674 (  5.219284)
Strftime        2.861623   0.740209   3.601832 (  3.603956)
---------------------------------------- total: 8.817506sec

                    user     system      total        real
Time#strftime   4.688617   0.847361   5.535978 (  5.539647)
Strftime        2.691129   0.852342   3.543471 (  3.546031)
```

Using following benchmark script:

```ruby
require "benchmark"
require_relative "./lib/fluent/load"

N = 1000

formatter = Strftime.new('%Y-%m-%d %H:%M:%S.%N %z')

event_times = []
1000.times do
  event_times << Fluent::EventTime.new(Time.now.to_i, (rand * 10**9).to_i)
end

Benchmark.bmbm do |x|
  x.report("Time#strftime") do
    N.times do
      event_times.each do |event_time|
        Time.at(event_time).strftime('%Y-%m-%d %H:%M:%S.%N %z')
      end
    end
  end
  x.report("Strftime") do
    N.times do
      event_times.each do |event_time|
        formatter.exec(Time.at(event_time))
      end
    end
  end
end
```