http://robots.thoughtbot.com/post/213842585/fundamentals-tdd-a-date-helper

date_time_helper.rb

module DateTimeHelper
  def format_date_range(date_range)
    first = date_range.first
    last  = date_range.last
    if same_day?(first, last)
      "#{first.to_s(:month_day)}, #{first.year}"
    elsif same_month?(first, last)
      "#{first.to_s(:month_day)}-#{last.day}, #{last.year}"
    else
      "#{first.to_s(:month_day)}-#{last.to_s(:month_day)}, #{last.year}"
    end
  end

  private

  def same_day?(date_one, date_two)
    date_one.day == date_two.day
  end

  def same_month?(date_one, date_two)
    date_one.month == date_two.month
  end
end

date_time_helper_test.rb

require 'test_helper'

class DateTimeHelperTest < ActionView::TestCase
  should "format date range on same day" do
    eight_oclock = DateTime.new(2009, 10, 12, 8)
    nine_oclock  = DateTime.new(2009, 10, 12, 9)
    date_range   = eight_oclock..nine_oclock
    expected     = "October 12, 2009"

    assert_equal expected, format_date_range(date_range)
  end

  should "format date range on different days of same month" do
    monday     = DateTime.new(2009, 10, 12)
    tuesday    = DateTime.new(2009, 10, 13)
    date_range = monday..tuesday
    expected   = "October 12-13, 2009"

    assert_equal expected, format_date_range(date_range)
  end

  should "format date range on days of different months" do
    october    = DateTime.new(2009, 10, 31)
    november   = DateTime.new(2009, 11, 1)
    date_range = october..november
    expected   = "October 31-November  1, 2009"

    assert_equal expected, format_date_range(date_range)
  end
end

index.html.erb

<%= format_date_range(course.date_range) %>

time_formats.rb

{ :short_date  => "%x",                 # 04/13/10
  :long_date   => "%a, %b %d, %Y",      # Tue, Apr 13, 2010
  :longer_date => "%B %d, %Y %H:%M %Z", # April 13, 2010 11:20
  :index       => "%Y/%m/%d %H:%M",     # 2010/04/13 11:20
  :standard    => "%B %d, %Y",          # April 13, 2010
  :month_day   => "%B %e",              # April 13
  :abbr_month  => "%b"                  # Apr
}.each do |key, value|
  ActiveSupport::CoreExtensions::Time::Conversions::DATE_FORMATS.update(
    key => value
  )
end