Stripe Billing Period Logic

billing_period.rb

class BillingPeriod
  attr_reader :start, :end

  def initialize(start_dt, end_dt)
    @start = start
    @end = end_dt
  end

  # the bill day can always be determined from any given period
  #   eg: if bill day is 1-28, the start and end will both always be on 28
  #   eg. if bill day is 31, either start or end will be 31 (we take the max)
  #      how stripe handles 31 bill day: jan 31, feb 28, mar 31, apr 30, ....
  def bill_day
    [start.day, self.end.day].max
  end

  def next
    self.class.new(self.end, add_month_preserving_bill_day(self.end))
  end

  def plus_months(n)
    period = self

    n.times do
      period = period.next
    end

    period
  end

  # would be more efficient to do some date arithmetic instead of looping, but usually only ever
  # going one or two months anyway, so who cares
  def period_at(time)
    raise StandardError.new("Error: tried to look up billing period for date in the past") if time < self.start
    per = self

    loop do
      if per.end > time
        return per
      end

      per = per.next
    end
  end

  # returns billing period starting at time, ending one month later. Mostly just a utility used in tests
  def self.starting_at(time)
    new(time, time + 1.month)
  end

  # build a billing period ending at the specified time (mostly used in tests)
  # eg. if Dec 31, would return Nov 30 - Dec 31. if Feb 28, would return Jan 28 - Feb 28
  def self.ending_at(time)
    start = time - 1.month
    start = start.change(day: [start.end_of_month.day, time.day].min)
    new(start, time)
  end

  private

  def add_month_preserving_bill_day(time)
    new_time = time + 1.month
    new_time.change(day: [new_time.end_of_month.day, bill_day].min)
  end
end

user.rb

class User < ApplicationRecord
  def period
    if subsc = stripe_subscription
      if subsc.current_period_start && subsc.current_period_end
        BillingPeriod.new(Time.at(subsc.current_period_start), Time.at(subsc.current_period_end))
      end
    end
  end
end