title: Activeadmin set default date
body: <p>When a new record is created I want to set the default date to a month ahead.</p>
  
  <p>With other fields the default is set in the migration, but where would you set it in this situation?</p>
  
  <p><img src="https://i.stack.imgur.com/p4x2u.png" alt="enter image description here"></p>
  
  <hr>
  
  <p><strong>Update</strong></p>
  
  <p>I tried to add it to</p>
  
  <p><strong>models/invoice.rb</strong></p>
  
  <pre><code> class Invoice &lt; ActiveRecord::Base
   before_create :set_due_date
  
   private
     def set_due_date
        self.due_date = DateTime.now + 30
     end
   end
  </code></pre>
  
  <p><strong>admin/invoice.rb</strong></p>
  
  <pre><code>form do |f|
     f.inputs "Options" do
        f.input :due_date, :as =&gt; :datepicker
     end
  end
  </code></pre>
  
  <p><strong>Migration</strong></p>
  
  <pre><code>create_table :invoices do |t|
     t.datetime :due_date
  end
  </code></pre>