MongoDB:查询日期范围内的集合

我在mongodb中有一个集合，其中包含此格式的记录

 db.userlogins.find().pretty()
{
        "_id" : ObjectId("519bb8c11f30bbf5bcba06f0"),
        "userId" : "sai",
        "created_at" : "2013-05-16 10:31:02.765"
}
{
        "_id" : ObjectId("519bb8cf1f30bbf5bcba06f1"),
        "userId" : "sai",
        "created_at" : "2013-05-17 10:31:02.765"
}
{
        "_id" : ObjectId("519bb8db1f30bbf5bcba06f2"),
        "userId" : "sai",
        "created_at" : "2013-05-18 10:31:02.765"
}
{
        "_id" : ObjectId("519bb8e71f30bbf5bcba06f3"),
        "userId" : "sai",
        "created_at" : "2013-05-19 10:31:02.765"
}
{
        "_id" : ObjectId("519bb8f21f30bbf5bcba06f4"),
        "userId" : "sai",
        "created_at" : "2013-05-20 10:31:02.765"
}
{
        "_id" : ObjectId("519bb8fc1f30bbf5bcba06f5"),
        "userId" : "sai",
        "created_at" : "2013-05-21 10:31:02.765"

我试图找出在一定范围内登录的用户

所以我试过了

db.userlogins.find({created_on: {$gte: "2013-05-17 10:31:02.765", $lt: "2013-05-21 10:31:02.765"}});

但我不知道为什么它不返回任何数据？？ 有人能帮忙吗这会有用的：

 db.userlogins.find({created_at: {$gte: "2013-05-17 10:31:02.765", $lt: "2013-05-21 10:31:02.765"}});