dater.rb

require 'date'
require 'time'

dates = File.open('dates.txt')
csv_array = Array.new

dates.readlines.each_with_index do |line, i|
  next if i == 0
  t = Time.parse(line)
  fixed_date = t.strftime("%F")
  csv_array << fixed_date
end

File.open('dates.csv', 'w') {|f| f.write(csv_array) }