random_date.rb

require 'date'

def random_date_between(args)
  start, finish = args[:start], args[:finish]
  start, finish = [start, finish].map do |datetime|
    case
    when datetime.is_a?(DateTime) then datetime.to_time.to_i
    when datetime.is_a?(Date) then DateTime.parse(datetime.to_s).to_time.to_i
    when datetime.is_a?(String) then DateTime.parse(datetime).to_time.to_i
    else
      raise "Cant use dates of #{datetime.class} class"
    end
  end
  DateTime.strptime((rand(finish - start) + start).to_s, "%s")
end

p random_date_between(start: "1970/1/1", finish: "2014/12/18")