Ruby on Rails: How to "un-do" timestamp if I uncheck a checkbox

I'm working on a list where a checkbox indicates completion of an item, and a completion_date is populated with the current date/time if the checkbox is checked. My method is:

def complete
  @task = Task.find(params[:id])
  if @task.complete == nil || @task.complete == false
    @task.update(complete: true)
    @task.update(completion_date: Date.today)
    redirect_to task_path
  else
    redirect_to task_path
  end

end

So if the box is checked, the date is filled. However, I want my users to be able to uncheck the checkbox, and therefore void the timestamp.

I'm a rails newbie. Any help greatly appreciated.