advancing_time_range.rb

class AdvancingTimeRange
  def initialize(time = '2009-03-01')
    @cursor = DateTime.parse(time)
  end

  def next(increment_by)
    range = @cursor..@cursor + increment_by
    @cursor = range.end
    range
  end
end

ascending_range.rb

class AscendingRange
  delegate :begin, :end, :to => @range

  def initialize(begin = 1, end = 50_000)
    @range = begin..end
  end

  def advance(by = 50_000)
    @range = (@range.begin + by)..(@range.end + by)
  end
end

month_span.rb

class MonthSpan
  def initialize(time = '2009-03-01')
    @start = DateTime.parse(time)
  end

  def range
    @start..@start + 1.month
  end

  def next
    @start = @start + 1.month
    range
  end
end