Collect start and end of month for given time range in Ruby (Rails)

collect_dates.rb

def start_and_end_of_month(date)
  [date.beginning_of_month,
   date.end_of_month]
end

def collect_dates(start_date, end_date = Time.zone.now.last_month.end_of_month, memo = [])
  if start_date < end_date
    memo << start_and_end_of_month(start_date)
    collect_dates(start_date.next_month, end_date, memo)
  else
    memo
  end
end

start_date = Time.zone.now.beginning_of_year
end_date = Time.zone.now.last_month.end_of_month

dates = collect_dates(start_date, end_date)
# => [[Sun, 01 Jan 2017 00:00:00 GMT +00:00, Tue, 31 Jan 2017 23:59:59 GMT +00:00],
#     [Wed, 01 Feb 2017 00:00:00 GMT +00:00, Tue, 28 Feb 2017 23:59:59 GMT +00:00],
#     [Wed, 01 Mar 2017 00:00:00 GMT +00:00, Fri, 31 Mar 2017 23:59:59 BST +01:00],
#     [Sat, 01 Apr 2017 00:00:00 BST +01:00, Sun, 30 Apr 2017 23:59:59 BST +01:00],
#     [Mon, 01 May 2017 00:00:00 BST +01:00, Wed, 31 May 2017 23:59:59 BST +01:00],
#     [Thu, 01 Jun 2017 00:00:00 BST +01:00, Fri, 30 Jun 2017 23:59:59 BST +01:00],
#     [Sat, 01 Jul 2017 00:00:00 BST +01:00, Mon, 31 Jul 2017 23:59:59 BST +01:00],
#     [Tue, 01 Aug 2017 00:00:00 BST +01:00, Thu, 31 Aug 2017 23:59:59 BST +01:00],
#     [Fri, 01 Sep 2017 00:00:00 BST +01:00, Sat, 30 Sep 2017 23:59:59 BST +01:00]]

dates.map do |start_date, end_date|
  [start_date.strftime('%Y-%m-%d'),
   end_date.strftime('%Y-%m-%d')]
end
# => [["2017-01-01", "2017-01-31"],
#     ["2017-02-01", "2017-02-28"],
#     ["2017-03-01", "2017-03-31"],
#     ["2017-04-01", "2017-04-30"],
#     ["2017-05-01", "2017-05-31"],
#     ["2017-06-01", "2017-06-30"],
#     ["2017-07-01", "2017-07-31"],
#     ["2017-08-01", "2017-08-31"],
#     ["2017-09-01", "2017-09-30"]]

str = dates.map do |start_date, end_date|
  [start_date.strftime('%Y-%m-%d'),
   end_date.strftime('%Y-%m-%d')].join(' ')
end