def time_rand from = 0.0, to = Time.now
  Time.at(from + rand * (to.to_f - from.to_f))
end

> time_rand
 => 1977-11-02 04:42:02 0100 
> time_rand Time.local(2010, 1, 1)
 => 2010-07-17 00:22:42 0200 
> time_rand Time.local(2010, 1, 1), Time.local(2010, 7, 1)
 => 2010-06-28 06:44:27 0200