Mongodb如何将时间戳转换为年月日日期

        Integer userId=aaa;
        GroupOperation groupOperation = Aggregation.group("day").sum("money").as("todayIncome").count().as("todayPayCount");
        Aggregation aggregation = Aggregation.newAggregation(
                Aggregation.match(Criteria.where("userId").is(userId)),
                project("userId","money").andExpression("{$dateToString: {date: { $add: {'$createTime', [0]} }, format: '%Y%m%d'}}", new Date(28800000)).as("day"),
                groupOperation,
                sort(Sort.Direction.ASC, "_id")
        );

import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.List;
 
import com.mongodb.BasicDBObject;
import com.mongodb.DB;
import com.mongodb.DBCollection;
import com.mongodb.DBCursor;
import com.mongodb.DBObject;
import com.mongodb.MongoClient;
import com.mongodb.ServerAddress;
 

public class MongDBTest {
	public static void main(String[] args) throws Exception {
		List<ServerAddress> list = new ArrayList<ServerAddress>();
//		连接数据库   ip 端口
		list.add(new ServerAddress("10.39.XXX.XXX", 27010));
		MongoClient mongoClient = new MongoClient(list);
		//数据库名称
	    DB psdoc = mongoClient.getDB("qa_db_center");
	    //表明
	    DBCollection collection=psdoc.getCollection("base_user_info");
	    
	    BasicDBObject queryObject = null; 
	    
	    // 时间查询    数据库看到的时间不是真实时间  加8小时后才是正确的时间
	    DBObject dbObject = new BasicDBObject();
	    String startDate = "2018-03-29 15:59:06";
	    String endDate = "2018-03-29 16:30:46";
	    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
	    dbObject.put("$gte", sdf.parse(startDate));
	    dbObject.put("$lte",  sdf.parse(endDate));
	    queryObject = new BasicDBObject();
	    queryObject.put("create_time",dbObject);
	    DBCursor find = collection.find(queryObject);
	     
	    while (find.hasNext()) {
	    	 DBObject next = find.next();
	    	 Object real_name = next.get("real_name");
	    	 Object mobile = next.get("mobile");
	    	 Object create_time = next.get("create_time"); 
	    	 String str = sdf.format(create_time);
	    	 System.out.println(real_name +"====="+mobile +"====="+str);
		}
	     System.out.println("结束");
	    
	}
}

db.getCollection('base_user_info').find({"create_time":{"$gte":ISODate("2018-03-29 07:59:06"),"$lte":ISODate("2018-03-29 08:30:46")}});