title: Mongoose date field - Set default to date.now + N days
body: <p>In a mongoose schema such as:  </p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>var EventSchema = new Schema({
   title: {
    type: String,
    default: '',
    trim: true,
    required: 'Title cannot be blank'
   },
   description: {
    type: String,
    default: '',
    trim: true
   },
   start: {
    type: Date,
    default: Date.now,
    required: 'Must have start date - default value is the created date'
   },
   end: {
    type: Date,
    default: Date.now + 7 Days, // Date in one week from now
    required: 'Must have end date - default value is the created date + 1 week'
   },
   tasks: [{
    type: Schema.ObjectId,
    ref: 'Task'
   }]
  });</code></pre>
  </div>
  </div>
  </p>
  
  <p>On the line for the "end" field the default date should set to +7 days.  I can add presave hook and set it there, but wondering if theres a way to do this inline in the default field.  </p>