duration_calc.rb

module DurationCalc
  def self.days_within(start, finish)
    durations_within(start, finish, 1.day)
  end

  # Calculates months between 2 gives dates. If 2 dates are of the same month returns 1
  def self.months_within(start, finish)
    durations_within(start, finish, 1.month)
  end

  private

  def self.durations_within(start, finish, duration)
    return durations_within(finish, start, duration) if start > finish

    count = 1

    # Yes we really need to loop here, to account for both DST and for leap year edge cases.
    while ((start += duration) <= finish)
      raise "Endless loop safegaurd #{count} #{start} #{finish}" if count > 5_000_000
      count += 1
    end

    count
  end
end