duration.rb

class Duration
  include Comparable

  attr_reader :milliseconds

  def initialize(value = 0)
    case value
    when Fixnum
      @milliseconds = value
    when String
      minutes, seconds = value.split(':').map(&:to_f)
      if seconds.nil?
        @milliseconds = minutes * 1000
      else
        @milliseconds = (minutes * 60 * 1000 + seconds * 1000).to_i
      end
    else
      raise ArgumentError
    end
  end

  def to_i
    milliseconds
  end

  def to_s
    seconds_string = sprintf('%.3f', seconds)

    if over_a_minute?
      "#{minutes}:#{seconds_string.rjust(6, '0')}"
    else
      seconds_string
    end
  end

  def <=>(other)
    milliseconds <=> other.milliseconds
  end

  private

  def seconds
    if over_a_minute?
      (milliseconds - 60_000).to_f / 1_000
    else
      milliseconds.to_f / 1_000
    end
  end

  def minutes
    if over_a_minute?
      milliseconds / 60_000
    end
  end

  def over_a_minute?
    milliseconds > 60_000
  end
end

timeable.rb

require 'active_support/concern'

module Timeable
  extend ActiveSupport::Concern

  included do
    composed_of :duration,
      mapping: %i(milliseconds),
      converter: :new
  end
end