Getting unix timestamp in seconds from MongoDB ISODate during aggregation

Question

I was looking for this, but I could not find anything useful to solve my case. What I want is to get the unix timestamp in seconds from the MongoDB ISODate during aggregation. The problem is that I can get the timestamp from ISODate, but this is in milliseconds. Therefore, I will need to cut out these milliseconds. I tried:

> db.data.aggregate([
    {$match: {dt:2}}, 
    {$project: {timestamp: {$concat: [{$substr: ["$md", 0, -1]}, '01', {$substr: ["$id", 0, -1]}]}}}
  ])

As you can see, I am trying to get the timestamp from "md" var, and also combine this timestamp with "01" and "id". The above code gives:

{
    "_id" : ObjectId("52f8fc693890fc270d8b456b"),
    "timestamp" : "2014-02-10T16:20:56011141"
}

Then I improved the command with

> db.data.aggregate([
    {$match: {dt:2}},
    {$project: {timestamp: {$concat: [{$substr: [{$subtract: ["$md", new Date('1970-01-01')]}, 0, -1]}, '01', {$substr: ["$id", 0, -1]}]}}}
  ])

Now I get:

{
    "_id" : ObjectId("52f8fc693890fc270d8b456b"),
    "timestamp" : "1392049256000011141"
}

I really need 1392049256011141, so without 3 extra 000. I tried subtracting $:

> db.data.aggregate([
    {$match: {dt:2}}, 
    {$project: {timestamp: {$concat: [{$substr: [{$divide: [{$subtract: ["$md", new Date('1970-01-01')]}, 1000]}, 0, -1]}, '01', {$substr: ["$id", 0, -1]}]}}}
  ])

I get:

{
    "_id" : ObjectId("52f8fc693890fc270d8b456b"),
    "timestamp" : "1.39205e+009011141"
}

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Answer 1

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db.data.aggregate([
  { "$project": {
    "timestamp": {
      "$subtract": [
        { "$divide": [
            { "$subtract": [ "$md", new Date("1970-01-01") ] },
            1000
        ]},
        { "$mod": [
          { "$divide": [
              { "$subtract": [ "$md", new Date("1970-01-01") ] },
              1000
          ]},
          1
        ]}
      ]
    }
  }}
])

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, , BSON date/datetime, timestamp. JavaScript :

var date = new Date()
( date.valueOf() / 1000 ) - ( ( date.valueOf() / 1000 ) % 1 )

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db.data.aggregate([
    { "$group": {
        "_id": {
          "$subtract": [
              { "$subtract": [ "$md", new Date("1970-01-01") ] },
              { "$mod": [
                  { "$subtract": [ "$md", new Date("1970-01-01") ] },
                  1000 * 60 * 60 * 24
              ]}
          ]
        },
        "count": { "$sum": 1 }
    }}
])

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