MongoDB $millisecond Operator

Example 1 — Sample Data

Imagine two sales with precise timestamps. We save them in a collection named sales.

db.sales.insertMany([
  {
    _id: 1,
    item: "abc",
    price: 10,
    quantity: 2,
    date: ISODate("2014-01-01T08:15:39.736Z")
  },
  {
    _id: 2,
    item: "xyz",
    price: 20,
    quantity: 1,
    date: ISODate("2014-01-01T08:15:39.000Z")
  }
]);

How the Data Is Stored in MongoDB

After the insert, MongoDB keeps two documents in sales. Run find() to see them:

db.sales.find()

[
  {
    _id: 1,
    item: "abc",
    price: 10,
    quantity: 2,
    date: ISODate("2014-01-01T08:15:39.736Z")
  },
  {
    _id: 2,
    item: "xyz",
    price: 20,
    quantity: 1,
    date: ISODate("2014-01-01T08:15:39.000Z")
  }
]

Read one document like this:

How It Works

1. Insert once. insertMany adds two sale documents to sales.
2. Dates are BSON Date. Use ISODate(...), not plain strings.
3. find() shows the same shape. Ready for $millisecond next.

Example 2 — Extract Milliseconds

Goal: read the 0–999 millisecond portion of each sale’s date.

db.sales.aggregate([
  {
    $addFields: {
      ms: { $millisecond: "$date" }
    }
  }
]);

{
  _id: 1,
  item: "abc",
  price: 10,
  quantity: 2,
  date: ISODate("2014-01-01T08:15:39.736Z"),
  ms: 736
}
{
  _id: 2,
  item: "xyz",
  price: 20,
  quantity: 1,
  date: ISODate("2014-01-01T08:15:39.000Z"),
  ms: 0
}

How It Works

1. Read the stored date. Document 1 has …39.736Z.
2. Extract ms. $millisecond: "$date" returns 736.
3. Whole seconds show 0. Document 2 ends in .000, so ms is 0.
4. Keep all fields. $addFields adds ms next to the original sale data.

Example 3 — Clock Parts Together

Same sales, but project hour, minute, second, and millisecond side by side.

db.sales.aggregate([
  {
    $project: {
      item: 1,
      hour: { $hour: "$date" },
      minutes: { $minute: "$date" },
      seconds: { $second: "$date" },
      milliseconds: { $millisecond: "$date" }
    }
  }
]);

{
  _id: 1,
  item: "abc",
  hour: 8,
  minutes: 15,
  seconds: 39,
  milliseconds: 736
}
{
  _id: 2,
  item: "xyz",
  hour: 8,
  minutes: 15,
  seconds: 39,
  milliseconds: 0
}

How It Works

1. Same stored dates. Still using the two sale timestamps from Example 1.
2. Split the clock. $hour, $minute, $second, and $millisecond each take one part.
3. UTC by default. 08:15:39.736Z → hour 8, minutes 15, seconds 39, ms 736.
4. $project keeps only listed fields (plus _id unless you turn it off).