Node + mongo-native: how to find and update value in collection?

I have user accounts, expiring by time. Time is stored in MongoDB as a long in milliseconds. Users are identified by _id. Here's the schema:

{
    "_id": {
        "$oid": "506a1438be4f73c11c000002"
    },
    "email": "admin",
    "password": "admin",
    "first_name": "admin",
    "last_name": "admin",
    "country": "USA",
    "group": "admin",
    "expires": 1349129272918
}

To add +N days to user accounts, I use find() and then update () (dont know why, but findAndUpdate can't get previous value):

app.post('/users/add', function(req,res){
    addDays(req.body.id, req.body.days, function(status){
        res.send(200);
    })
});

function addDays(id, days, callback) {
    var obj_id = BSON.ObjectID.createFromHexString(id);
    db.collection("users", function(error, collection) {
        collection.find({_id: obj_id}, function(err, user) {
            var oldexp = user.expires;
            if (oldexp < new Date().getTime() ) {
                var newexp = new Date().getTime() + (86400000*days);
            } else {
                var newexp = oldexp + (86400000*days);
            }
            collection.update({_id: obj_id}, {
                $set: {
                    "expires": parseInt(newexp)
                }
            }, function(err, result) {
                callback('ok');
            });
        });
    });
}

To launch update days function, I use JS on frontend:

  $(".addDays").live("click", function(){
    var that = $(this);
    var id = that.closest('.row').find('input[name=id]').val();
    alert(id);
    $.post('/users/add', {
        "id": id,
        "days": 10
      }, function(data){
          alert(data);
      });
  });

Var id is a string-decoded ObejctID (like "506a1438be4f73c11c000002"), for every user it's stored in the nearest input(name="id").

After I try to add days, even I get OK, my collection becomes damaged. Does anyone know why? Thanks!