How to calculate timestamp difference in mongodb (in hours)?

Question

Here's my data:

[
   {
       id: 1, 
       starttime: ISODate("2015-08-24T00:00:00.000Z"), 
       endtime: ISODate("2015-08-24T07:00:00.000Z")
   },
   {
       id: 2, 
       starttime: ISODate("2015-08-24T20:00:00.000Z"), 
       endtime: ISODate("2015-08-25T01:00:00.000Z")
   }
]

can I make a mongodb query to display duration (or in this case a difference operation) of starttime and endtime with results like:

[ {id:1, duration: 7}, {id: 2, duration: 5}]

Notice that timestamp can have different date so $hour(aggregation pipeline) might not work. Can anyone help ? Thank you

Answer 1

db.collectionname.aggregate([
  {$project: {
      duration: {$divide: [{$subtract: ["$endtime", "$starttime"]}, 3600000]}
  }}
])

Answer 2

Starting in Mongo 5.0, the $dateDiff aggregation operator perfectly fits your use case of producing the difference between two dates:

// { _id: 1, start: ISODate("2015-08-24T00:00:00Z"), end: ISODate("2015-08-24T07:00:00Z") }
// { _id: 2, start: ISODate("2015-08-24T20:00:00Z"), end: ISODate("2015-08-25T01:00:00Z") }
db.collection.aggregate([
  { $project:
    { duration:
      { $dateDiff: { startDate: "$start", endDate: "$end", unit: "hour" } }
    }
  }
])
// { "_id" : 1, "duration" : 7 }
// { "_id" : 2, "duration" : 5 }