simple timespan class for operations on time span

timespan.rb

# usage Timespan.to_words (Time.now + 10)
# usage t_one = Time.now
# usage t_two = Time.now + 100
# usage Timespan.to_words t_one
# usage Timespan.to_words t_one..t_two
class Timespan

  def self.to_words(time)
    unless (time.is_a?(Time) or (time.is_a?(Range) and (time.first.is_a?(Time) and time.last.is_a?(Time))) )
      raise 'Time should be a Time/(Range in Time) object #{time.class} given'
    end
    diff  = time.is_a?(Range) ? time.first.to_i - time.last.to_i : Time.now.to_i - time.to_i
    diff_to_words(diff)
  end

  def self.diff_to_words(diff)
    diff_noun = noun(diff)
    diff_count = count(diff)
    "about #{diff_count} #{infect(diff_noun, diff_count)} %s" % (diff >0 ? 'ago' : 'from now')
  end

  private

  def self.infect(word, count)
    count.abs > 1 ? word + "'s" : word
  end

  def self.count(num)
    num = num.abs
    ret_val = num
    [60,60,24,7,4,12].each do |n|
      break if (num = num/n) == 0
      ret_val = num % n
    end
    ret_val
  end

  def self.noun(diff)
    limits = [["minute", 60], ["hour", 3600], ["day", 86400], ["week", 604800], ["month", 2678400], ["year", 31622400]]
    ret_val = 'second'
    limits.each{|val| ret_val = val[0] if val[1] < diff.abs }
    ret_val
  end
end