help:How to set default value for a column use of migration?

hi,all..
Here i want to use of migration to create a table and set a defult
value for a column..but i cant't finished..

mycode:
class CreateMessages < ActiveRecord::Migration
  def self.up
    create_table :messages do |t|
      t.column :name, :string
      t.column :content, :text
      t.column :posttime, :datetime, :default => "now()"
    end
  end

  def self.down
    drop_table :messages
  end
end

it can't work sucessfully.And the error like this:
(in G:/Jc_RubyonRails/InstantRails/rails_apps/guestbook)
== CreateMessages: migrating

Hi, you can do one of the following:

t.column :posttime, :datetime, :default => Time.now

or

t.column :created_at, :datetime

Good luck,

-Conrad