Reading a file line by line and parsing each line.

date_parser.rb.rb

require "date"

new_dates = File.open("new_dates.txt","w")

File.open("old_dates.txt","r") do |f| 
  while line = f.gets
    newdate = DateTime.parse(line)
    new_dates << newdate.strftime("%Y-%m-%d %H:%M:%S") << "\n"
  end
end

new_dates.close