original.rb

def display_date_or_range(event, sup = true, html = true, short = false)
  return if event.start_date.blank? and event.end_date.blank?
  if event.single_day?
    if short
      out = event.start_date.strftime("%b")
    else
      out = event.start_date.strftime("%B")
    end

    out << " "
    if sup === false
      out << event.start_date.day.ordinalize
    else
      out << ordinalize_with_sup(event.start_date)
    end
    out << ", #{event.start_date.year}"
    return out.html_safe
  end

  if event.multi_day?
    out = ''
    if short
      out << event.start_date.strftime("%b ")
    else
      out << event.start_date.strftime("%B ")
    end
    if sup === false
      out << event.start_date.day.ordinalize
    else
      out << ordinalize_with_sup(event.start_date)
    end
    if event.spans_different_years?
      out << ", #{event.start_date.year}"
    end
    if html
      out << '&ndash;'
    else
      out << HTMLEntities.new.decode('&ndash;')
    end
    if event.spans_different_months? or (!event.spans_different_months? && event.spans_different_years?)
      out << event.end_date.strftime("%B ")
    end
    if sup === false
      out << event.end_date.day.ordinalize
    else
      out << ordinalize_with_sup(event.end_date)
    end
    out << ", #{event.end_date.year}"
    out.html_safe
  end
end

refactor.rb

class DateOrRange
  attr_accessor :start_date, :end_date, :options

  def initialize(options = {})
    @start_date = options.delete(:start_date)
    @end_date = options.delete(:end_date)
    @end_date = nil if @start_date == @end_date
    @options = options
  end

  def single_day?
    start_date.present? && end_date.blank?
  end

  def start_date_month
    return if start_date.blank?
    start_date.strftime("%B")
  end

  def start_date_month_word
    start_date_month
  end

  def start_date_number
    start_date.day.ordinalize
  end

  def start_date_year
    start_date.year
  end

  def start_date_year_word
    start_date_year if end_date.blank? || (start_date_year != end_date_year)
  end

  def start_date_part
    return if start_date.blank?
    [
      start_date_month_word,
      "#{start_date_number}#{',' if start_date_year != end_date_year}",
      (start_date_year_word)
    ].reject(&:nil?).join(' ')
  end

  def end_date_month
    end_date.strftime("%B")
  end

  def end_date_month_word
    end_date_month if (end_date_month != start_date_month) || (start_date_year != end_date_year)
  end

  def end_date_number
    end_date.day.ordinalize
  end

  def end_date_year
    end_date.year if end_date.present?
  end

  def end_date_part
    return if end_date.blank?
    [
      end_date_month_word, 
      "#{end_date_number},",
      end_date_year
    ].reject(&:nil?).join(' ')
  end

  def to_s
    [
      start_date_part,
      end_date_part
    ].reject(&:nil?).join('–')
  end
end

I like reject(&:blank?) because it also copes with empty strings.

> ["", "Last"].compact.join(" ")
=> " Last"

whereas:

> ["", "Last"].reject(&:blank?).join(" ")
=> "Last"

I've sometimes done it like this:

> ["", "Last"].compact.join(" ").strip
=> "Last"

That doesn't work as well with more than two words where one in the middle is of zero length (you get a double space).

To my shame, I've also used the very inelegant .select {|w| !w.blank? } so I'm glad to have been reminded about reject.

See also my gist about titleizing names, which could incorporate some of these ideas too.