class Holidays::Finder::Context::DatesDriverBuilder

def add_border_months(current_date, dates_driver)

to me at this stage.
we are increasing runtimes slightly but improving accuracy, which is more important
dates falling on 2/1 but being observed on 1/31). By including the additional months
additional months in an attempt to catch month-spanning date situations (i.e.
As part of https://github.com/holidays/holidays/issues/146 I am returning 
def add_border_months(current_date, dates_driver)
  if current_date.month == 1
    dates_driver[current_date.year] << 2
    prev_year = current_date.year - 1
    dates_driver[prev_year] = [] unless dates_driver[prev_year]
    dates_driver[prev_year] << 12
  elsif current_date.month == 12
    dates_driver[current_date.year] << 11
    next_year = current_date.year + 1
    dates_driver[next_year] = [] unless dates_driver[next_year]
    dates_driver[next_year] << 1
  else
    dates_driver[current_date.year] << current_date.month - 1 << current_date.month + 1
  end
  dates_driver
end

def call(start_date, end_date) 

def call(start_date, end_date)
  dates_driver = {}
  (start_date..end_date).each do |date|
    dates_driver[date.year] = [] unless dates_driver[date.year]
    dates_driver[date.year] << date.month
    dates_driver = add_border_months(date, dates_driver)
  end
  clean(dates_driver)
end

def clean(dates_driver) 

def clean(dates_driver)
  dates_driver.each do |year, months|
    # Always add variable month '0' for proc calc purposes. For example, 'easter' lives in
    # 'month 0' but is vital to calculating a lot of easter-related dates.
    dates_driver[year] << 0
    dates_driver[year].uniq!
    dates_driver[year].sort!
  end
  dates_driver
end