time_object.change(:day => date_object.day, :month => date_object.month, :year => date_object.year)

def before_validation
    logger.info("show_time = " + self.show_time.to_s)
    self.show_time.change(:year => 2012)
    self.show_time.change(:minute => 30)
    logger.info("show_time = " + self.show_time.to_s)
  end

>> t = Time.now
=> Thu Apr 09 21:03:25 +1000 2009
>> t.change(:year => 2012)
=> Mon Apr 09 21:03:25 +1000 2012
>> t
=> Thu Apr 09 21:03:25 +1000 2009
>> t = t.change(:year => 2012)
=> Mon Apr 09 21:03:25 +1000 2012
>> t
=> Mon Apr 09 21:03:25 +1000 2012

self.show_time = self.show_time.change(:year => 2012)