public void SerializeProperty(
     BsonWriter bsonWriter,
     object obj,
     BsonPropertyMap propertyMap
 )
 {
     var value = (BsonTimestamp) propertyMap.Getter(obj);
     if (value == null) {
         bsonWriter.WriteNull(propertyMap.ElementName);
     } else {
         bsonWriter.WriteTimestamp(propertyMap.ElementName, value.Value);
     }
 }