如何在mongodb中减去两个日期时间?

I have used aggregate function.

我用了聚合函数。

db.checkins.aggregate([
       {$match: {checkinType: "Beacon",
               "associationIds.organizationId":"af39bc69-1938-4149",
               "checkinData.time": {"$gte": new Date("2018-01-18T18:30:00.000Z"), 
                                   "$lt": new Date("2018-01-19T18:30:00.000Z")}
                }
        },
       {"$sort":{"checkinData.time":-1}},
       {$group: {"_id":
                    {"orgId":"$asst.organizationId", "userId":"$asst.userId"},
                    "lastSeen":{"$first":"$checkinData.time"},
                   "firstSeen":{"$last":"$checkinData.time"},
               }
       }, 
      {"$project":{"_id":1,"lastSeen":1, "firstSeen":1, 
                  totalHourSpent:{$subtract: ["$lastSeen","$firstSeen"]}}}, 
  ])
db