title: Set TTL doesnt work in Mongodb c#
body: <p>I am using MongoDb in c# . I insert a  Student Like below </p>
  
  <pre><code> _client = new MongoClient(new MongoClientSettings
          {
              Server = new MongoServerAddress("172.1.9.8", 27017)
          });
          _database = _client.GetDatabase("MongoSample");
  
          Student st=new Student()
          {
              Name = "Ali",
              Family = "Valid",
              Age = 25,
              expireAt = DateTime.Now.AddSeconds(30)
          };
    var collection = _database.GetCollection&lt;Student&gt;("Students");
       collection.InsertOne(st);
  </code></pre>
  
  <p>I want to set a <code>TTL</code>  to Remove data after some seconds , I try to do that like this :</p>
  
  <pre><code> var collection = _database.GetCollection&lt;Student&gt;("Students");
          var indexs = collection.Indexes.CreateOne(Builders&lt;Student&gt;.IndexKeys.Ascending("Name"),
              new CreateIndexOptions { ExpireAfter = new TimeSpan(0, 0, 10) });
  
          collection.InsertOne(st);
  </code></pre>
  
  <p>but it does not work .
  my model is :</p>
  
  <pre><code> public class Student
  {
      public string Name { get; set; }
      public string Family { get; set; }
      public int Age { get; set; }
      [BsonElement("expireAt")]
      public DateTime expireAt { get; set; }
  }
  </code></pre>
  
  <p>Note : and I have problem with <code>DateTime</code> , stored data  in db is different with Set Date in <code>Visual Studio</code> </p>