{
   "_id":"5c59c35d8610f702d00e6f70",
   "ipAddress":"50.116.14.48",
   "startTime":"2018-02-06T12:01:59.000Z",
   "endTime":"2018-02-06T12:31:00.000Z",
}

[
   {
      "occurrences":1,
      "startWindow":"2018-02-06T12:00:00.000Z",
      "endWindow":"2018-02-06T12:15:00.000Z"
   },
   {
      "occurrences":1,
      "startWindow":"2018-02-06T12:15:01.000Z",
      "endWindow":"2018-02-06T12:30:00.000Z"
   },
   {
      "occurrences":1,
      "startWindow":"2018-02-06T12:30:01.000Z",
      "endWindow":"2018-02-06T12:45:00.000Z"
   }
]

db.col.aggregate([
    {
        $project: {
            startTime: { $toLong: { $toDate: "$startTime" } },
            endTime: { $toLong: { $toDate: "$endTime" } }
        }
    },
    {
        $project: {
            ranges: [
                {
                    start: { $subtract: [ "$startTime", { $mod: [ "$startTime", 900000 ] } ] },
                    end: { $add: [ { $subtract: [ "$startTime", { $mod: [ "$startTime", 900000 ] } ] }, 900000 ] }
                },
                {
                    start: { $subtract: [ "$endTime", { $mod: [ "$endTime", 900000 ] } ] },
                    end: { $add: [ { $subtract: [ "$endTime", { $mod: [ "$endTime", 900000 ] } ] }, 900000 ] }
                }
            ]
        }
    },
    {
        $unwind: "$ranges"
    },
    {
        $group: {
            _id: "$ranges",
            count: { $sum: 1 }
        }
    },
    {
        $project: {
            _id: 0,
            occurences: "$count",
            startWindow: { $toString: { $toDate: "$_id.start" } },
            endWindow: { $toString: { $toDate: "$_id.end" } }
        }
    }
])