Convert date from milliseconds to ISODate object

Question

I am trying to aggregate records in a MongoDB collection by hour and need to convert date stored as timestamp (milliseconds) to ISODate so that I can use aggregate framework's built-in date operators ($hour, $month, etc.)

Records are stored as

{ 
"data" : { "UserId" : "abc", "ProjId" : "xyz"}, 
"time" : NumberLong("1395140780706"),
"_id" : ObjectId("532828ac338ed9c33aa8eca7") 
} 

I am trying to use an aggregate query of following type:

db.events.aggregate(
    { 
       $match : { 
         "time" : { $gte : 1395186209804, $lte : 1395192902825 } 
       } 
    }, 
    { 
       $project : {
         _id : "$_id", 
         dt : {$concat : (Date("$time")).toString()} // need to project as ISODate
       } 
    },
    // process records further in $project or $group clause
)

which produces results of the form:

{
    "result" : [
        { 
            "_id" : ObjectId("5328da21fd207d9c3567d3ec"), 
            "dt" : "Fri Mar 21 2014 17:35:46 GMT-0400 (EDT)" 
        }, 
        { 
            "_id" : ObjectId("5328da21fd207d9c3567d3ed"), 
            "dt" : "Fri Mar 21 2014 17:35:46 GMT-0400 (EDT)" 
        }, 
            ... 
} 

I want to extract hour, day, month, and year from the date but since time is projected forward as string I am unable to use aggregate framework's built-in date operators ($hour, etc.).

How can I convert time from milliseconds to ISO date to do sometime like the following:

db.events.aggregate(
    {
        $match : { 
            "time" : { $gte : 1395186209804, $lte : 1395192902825 } 
        }
    },
    {
        $project : {
            _id : "$_id",
            dt : <ISO date from "$time">
        }
    },
    { 
        $project : {
            _id : "$_id",
            date : { 
                hour : {$hour : "$dt"} 
            }
        }
    }
)

Answer 1

Actually, it is possible, the trick is to add your milliseconds time to a zero-milliseconds Date() object using syntax similar to:

dt : {$add: [new Date(0), "$time"]}

I modified your aggregation from above to produce the result:

db.events.aggregate(
    {
        $project : {
            _id : "$_id",
            dt : {$add: [new Date(0), "$time"]}
        }
    },
    { 
        $project : {
            _id : "$_id",
            date : { 
                hour : {$hour : "$dt"} 
            }
        }
    }
);

The result is (with one entry of your sample data):

{
  "result": [
    {
      "_id": ObjectId("532828ac338ed9c33aa8eca7"),
      "date": {
        "hour": 11
      }
    }
  ],
  "ok": 1
}