how to transfer this code to model?

I have a code which calculates a parameter in the create action and update action differently. Here expiry_time is the column in the db and expiry_duration is a virtual parameter.

class SomeController
  def create
    params[:model][:expiry_time] = Time.now + params[:model][:expiry_duration].to_i
    @model = Model.new(param开发者_JAVA技巧s[:model])
    if @model.save
      redirect_to ..
    else
      render ..
    end
  end

  def update
    @model = Model.find(params[:id])
    params[:model][:expiry_time] = @model.created_at + params[:model][:expiry_duration].to_i
    if @model.update_params(params[:model])
      redirect_to ..
    else
      render ..
    end
  end
end

I was thinking of moving the calculation part to model in a before_save function. Should I check for the id there to decide if it is a new record or an existing one like this?

class Model
  before_save :update_expiry_time

  def update_expiry_time
    start_time = id ? created_at : Time.now
    expiry_time = start_time + expiry_duration.to_i 
  end

  def expiry_duration
    expiry_time - created_at
  end
end

What do you think?

use new_record?, or use :on => :create to call a different funciton