Нашел вот это: http://www.techotopia.com/index.php/Working_with_Dates_and_Times_in_Ruby
Кажется, что вы ищете.
require 'date'
today = DateTime.now
=> #<DateTime: 441799066630193/180000000,-301/1440,2299161>
birthday = Date.new(2008, 4, 10)
=> #<Date: 4909133/2,0,2299161>
days_to_go = birthday - today
time_until = birthday - today
=> Rational(22903369807, 180000000)
time_until.to_i # get the number of days until my birthday
=> 127
hours,minutes,seconds,frac = Date.day_fraction_to_time(time_until)
[3053, 46, 57, Rational(1057, 180000000)]
puts "It is my birthday in #{hours} hours, #{minutes} minutes and #{seconds} seconds (not that I am counting)"
It is my birthday in 3053 hours, 46 minutes and 57 seconds (not that I am counting)