# Examples:
    #   Post.by_day
    #   Post.by_day(Time.yesterday)
    #   Post.by_day("next tuesday")
    def by_day(time = Time.zone.now, options = {}, &block)
      time = parse(time)
      by_star(time.utc.beginning_of_day, time.utc.end_of_day, options, &block)
    end
    alias_method :today, :by_day
    
    # Examples:
    #   Post.yesterday
    #   # 2 days ago:
    #   Post.yesterday(Time.yesterday)
    #   # day before next tuesday
    #   Post.yesterday("next tuesday")
    def yesterday(time = Time.zone.now, options = {}, &block)
      time = parse(time)
      by_day(time.advance(:days => -1), options, &block)
    end
    
    # Examples:
    #   Post.tomorrow
    #   # 2 days from now:
    #   Post.tomorrow(Time.tomorrow)
    #   # day after next tuesday
    #   Post.tomorrow("next tuesday")
    def tomorrow(time = Time.zone.now, options = {}, &block)
      time = parse(time)
      by_day(time.advance(:days => 1), options, &block)
    end