Ruby => 日付時刻

require "date"

# 2017/8/19 
start_date = Date.new(2017,8,19)

   => #<Date: 2017-08-19 ((2457985j,0s,0n),+0s,2299161j)>


#today
end_date  = Date.today 

  => #<Date: 2017-11-03 ((2458061j,0s,0n),+0s,2299161j)>  


# diff
(end_date -  start_date).to_i 

  => 76


# to_s
end_date.to_s 

  => "2017-11-03"


start_date.to_s 

  => "2017-08-19"


# yyyy_MMdd_HHmmss
end_date.strftime('%Y_%m%d_%H%M%S') 

  => "2017_1103_000000"


start_date.strftime('%Y_%m%d_%H%M%S') 

  => "2017_0819_000000"