DateTimeの差を秒単位で取る

gistfile1.rb

# coding: utf-8

require "date"

a = DateTime.new(2012, 1, 31, 23, 59, 55, "+09:00")
b = DateTime.new(2012, 2,  1,  0,  0,  0, "+09:00")

# (DateTime - DateTime)はRationalになるので、24 * 60 * 60をかけてto_iして秒単位に変換する
(b - a)                       # => (1/17500)
(b - a).class                 # => Rational
(b - a) * 24 * 60 * 60        # => (5/1)
((b - a) * 24 * 60 * 60).to_i # => 5

# DateTimeをTimeに変換してto_iしたものを引く
b.to_time                         # => 2012-02-01 00:00:00 +0900
a.to_time                         # => 2012-01-31 23:59:55 +0900
b.to_time.class                   # => Time
a.to_time.class                   # => Time
b.to_time.to_i                    # => 1328054400
a.to_time.to_i                    # => 1328054395
(b.to_time.to_i - a.to_time.to_i) # => 5