For recurring billing.

Bill.rb

# Calculates billable day-of-month numbers at the end
# of a month with fewer than 31 days.
#
# eg. billable_days Date.new(2012, 02, 29) => 29, 30, 31
#
def billable_days(date = Date.today)
  if date.day == date.end_of_month.day
    (date.day..31)
  else
    date.day
  end
end