title: Validate date values in Meteor AutoForm SimpleSchema
body: <p>I have the following Schema:</p>
  
  <pre><code>Dates.attachSchema(new SimpleSchema({
      description: {
          type: String,
          label: "Description",
          max: 50
      },
      start: {
          type: Date,
          autoform: {
              afFieldInput: {
                  type: "bootstrap-datepicker"
              }
          }
      },
      end: {
          type: Date,
          autoform: {
              afFieldInput: {
                  type: "bootstrap-datepicker"
              }
          }
      }
  }));
  </code></pre>
  
  <p>How can I validate that the <code>end</code> date is not before <code>start</code>? I am using MomentJS to handle date types, however my main problem is how I can access other attributes in the <code>custom</code> function.</p>
  
  <p>For instance:</p>
  
  <pre><code>end: {
     type: Date,
     autoform: {
         afFieldInput: {
             type: "bootstrap-datepicker"
         }
     },
     custom: function() {
         if (moment(this.value).isBefore(start)) return "badDate";
     }
  }
  </code></pre>
  
  <p>How can I access <code>start</code>?</p>
  
  <p>Furthermore, how can I validate if the <code>start</code> + <code>end</code> date combination is <strong>unique</strong>, meaning there is no document saved in my database which has the exact same <code>start</code> and <code>end</code> date?</p>