class AddUserTimeZones < ActiveRecord::Migration
  def self.up
    add_column :users, :time_zone, :string
  end

  def self.down
    remove_column :users, :time_zone
  end
end

class AddUserTimeZones < ActiveRecord::Migration
  def self.up
    add_column :users, :time_zone, :string, :default => "UTC"
  end

  def self.down
    remove_column :users, :time_zone
  end
end

Fix migration (add default time zone)
Fix migration (add default time zone)
