title: How to subtract two date time in mongodb
body: <p>I have used aggregate function.</p>
  
  <pre><code>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"]}}}, 
    ])
  </code></pre>
  
  <blockquote>
    <p>When I performed this Query mongo return <strong>totalHourSpent</strong> in milisecond which is given below.</p>
  </blockquote>
  
  <pre><code>{
    "_id" : {
          "orgId" : "af39bc69-1938-4149-b9f7-f101fd9baf73",
          "userId" : "34adb4a0-0012-11e7-bf32-cf79d6b423e9"
    },
   "lastSeen" : ISODate("2018-01-19T18:49:52.242+05:30"),
   "firstSeen" : ISODate("2018-01-19T10:08:21.026+05:30"),
   "totalHourSpent" : NumberLong("31291216")
  },
  {
    "_id" : {
         "orgId" : "af39bc69-1938-4149-b9f7-f101fd9baf73",
         "userId" : "679416b0-3f88-11e7-8d27-77235eb1ba9b"
     },
     "lastSeen" : ISODate("2018-01-19T20:51:30.946+05:30"),
     "firstSeen" : ISODate("2018-01-19T11:07:44.256+05:30"),
     "totalHourSpent" : NumberLong("35026690")
   },
  </code></pre>
  
  <blockquote>
    <p>How to calculate <strong>totalHourSpent</strong> in hour. Thanks in advance. </p>
  </blockquote>