$ irb
> require 'activesupport'
=> true
> time = Time.mktime 2010, 8, 29
=> 2010-08-29 00:00:00 +0900
> time = time.months_since 1
=> 2010-09-29 00:00:00 +0900
> time = time.months_since 1
=> 2010-10-29 00:00:00 +0900
> time = time.months_since 1
=> 2010-11-29 00:00:00 +0900
> time = time.months_since 1
=> 2010-12-29 00:00:00 +0900

これではどう?