Как то так?
irb(main):001:0> File.mtime("file")
=> 2010-10-08 17:56:10 +0800
irb(main):002:0> File.mtime("file").year
=> 2010
irb(main):003:0> File.mtime("file").month
=> 10
irb(main):004:0> File.mtime("file").day
=> 8
Аналогично с датой
irb(main):001:0> require 'date'
=> true
irb(main):002:0> Date.today
=> #<Date: 2010-10-11 (4910961/2,0,2299161)>
irb(main):003:0> Date.today.year
=> 2010
irb(main):004:0> Date.today.month
=> 10
irb(main):005:0> Date.today.day
=> 11
Или вы можете использовать strftime
irb(main):001:0> File.mtime("file").strftime("%Y-%m-%d")
=> "2010-10-08"