In MongoDB, the $toDate aggregation operator converts a value to a date

$toDate takes any valid expression. If the expression can’t be converted to a date, it returns an error. If the expression is null or missing, it returns null.

Example

Suppose we have a collection called samples with the following document:

{
	"_id" : ObjectId("60066f1cc8eb4369cf6ad9c9"),
	"double" : 1613372035500.75,
	"decimal" : NumberDecimal("1613372035500.75"),
	"long" : NumberLong("1613372035500"),
	"string" : "2021-02-15 06:53:55"
}

We can use the following query to return each of those values as a Date.

db.samples.aggregate(
  [
    {
      $project:
        {
          _id: 0,
          "ObjectId": { $toDate: "$_id" },
          "double": { $toDate: "$double" },
          "decimal": { $toDate: "$decimal" },
          "long": { $toDate: "$long" },
          "string": { $toDate: "$string" }
        }
    }
  ]
).pretty()

Result:

{
	"ObjectId" : ISODate("2021-01-19T05:33:16Z"),
	"double" : ISODate("2021-02-15T06:53:55.500Z"),
	"decimal" : ISODate("2021-02-15T06:53:55.500Z"),
	"long" : ISODate("2021-02-15T06:53:55.500Z"),
	"string" : ISODate("2021-02-15T06:53:55Z")
}

As expected, all fields are returned with their corresponding values converted to Date values.

You may notice that the ObjectId field returns a different date to the others. This is because the _id field’s value reflects the date that the document was created, whereas the other dates were just arbitrary dates created by me. More about the ObjectId below.

Input Types

The $toDate operator accepts the following input types:

• Double
• Decimal
• Long
• String (must be a valid date string)
• ObjectId

More about these below.

Numeric Types

When using the numeric types (Double, Decimal, and Long), $toDate returns a date that corresponds to the number of milliseconds represented by the value. For Double and Decimal, this is the truncated value. The $toDate operator interprets numeric values as follows:

• A positive value corresponds to the number of milliseconds since Jan 1, 1970.
• A negative value corresponds to the number of milliseconds before Jan 1, 1970.

String Types

When using a string type, the string must be a valid date string. For example:

• "2021-02-15"
• "15 February 2021"
• "2021-02-15 06:53:55"
• "2021-02-15 06:53:55+1100"

You can’t use incomplete dates or invalid values such as "Monday", "March", "2020", etc.

ObjectId

ObjectId values are 12 byte hexadecimal values that consist of:

• A 4 byte timestamp value, representing the ObjectId’s creation, measured in seconds since the Unix epoch.
• A 5 byte is a random value
• A 3 byte incrementing counter, initialised to a random value.

The $toDate operator returns the timestamp portion as a Date.