title: How to store TimeStamp? Is there automatic field for CreatedOn
body: <p>I am new to mongoDB. I have following code create collection and document But there is no timestamp field. Is there any way to insert <code>created_on</code> automatically in documents.</p>
  
  <pre><code>db.createCollection("countries"); //Created collection
  db.countries.insert({"short_name":"AF","country_name":"Afganistan"});  //Created document
  db.countries.find()
  { "_id" : ObjectId("53d1d5bcb8173a625961ff34"), "short_name" : "AF", "country_name" : "Afganistan" }
  </code></pre>
  
  <p>What I am really after is assigning a default "created_on" to my documents using Cake PHP. So how do you do that?</p>