gistfile1.rb

#!/usr/bin/env ruby

require 'benchmark'
require 'ostruct'

unlocked = 100000.times.map { o = OpenStruct.new; o.timestamp = Time.now - Kernel.rand(10000000); o } ; nil

method1 = lambda {
  achievements_per_day = Hash.new(0)
  unlocked.each do |achievement|
    achievements_per_day[achievement.timestamp.strftime("%D")] += 1 if achievement.timestamp
  end
}

method2 = lambda {
  achievements_per_day = unlocked.map(&:timestamp).compact.reduce(Hash.new(0)) { |accum, tstamp| accum[tstamp.strftime("%D")] += 1; accum }
}

avg_time = lambda { |n, fn|
  n.times.map { Benchmark.realtime(&fn) }.reduce(:+) / n.to_f
}

puts avg_time[15, method1] # 0.9437
puts avg_time[15, method2] # 0.9125