8桁の数字で素数となる日付

primedays.rb

require 'prime'
require 'date'

def primedays(year)
  Array(Date.new(year,1,1)..Date.new(year,12,31)).flat_map {|d|
    Prime.prime?(d.strftime('%Y%m%d').to_i) ? [d.to_s] : []
  }
end

primedays(2014) 
# => ["2014-02-01","2014-02-13","2014-03-01","2014-03-03","2014-03-27","2014-03-31","2014-04-11","2014-04-23","2014-04-29","2014-06-09","2014-06-13","2014-06-19","2014-08-07","2014-08-23","2014-08-29","2014-08-31","2014-09-07","2014-09-09","2014-09-27","2014-10-03","2014-10-21","2014-11-01","2014-11-11","2014-12-01","2014-12-03","2014-12-07","2014-12-21"]