In MongoDB, the Date() method returns a date either as a string or as a Date object.

• When you call it as Date(), it returns the current date as a string in the mongo shell.
• When you call it as new Date(), it returns the current date as a Date object.

You can also provide a specific date as an argument to new Date(), so that it uses that date.

Example – Date()

Here’s an example of inserting a document into a collection, and using Date() as the value for one of the fields:

db.dogs.insert(
  { 
    "_id" : 1,
    "name" : "Fetch", 
    "lastModified" : Date()
  }
)

After running that code, we can take a look at the resulting document:

db.dogs.find().pretty()

Result:

{
	"_id" : 1,
	"name" : "Fetch",
	"lastModified" : "Wed Jan 27 2021 10:15:53 GMT+1000 (AEST)"
}

We can see that the lastModified date field uses a date string as its value.

Example – new Date()

As mentioned, if you want to return a date object instead of a date string, use new Date().

Example:

db.dogs.insert(
  { 
    "_id" : 2,
    "name" : "Wag", 
    "lastModified" : new Date()
  }
)

Simply prepending Date() with new results in a Date object rather than a date string.

Now let’s check the collection of documents again:

db.dogs.find().pretty()

Result:

{
	"_id" : 1,
	"name" : "Fetch",
	"lastModified" : "Wed Jan 27 2021 10:15:53 GMT+1000 (AEST)"
}
{
	"_id" : 2,
	"name" : "Wag",
	"lastModified" : ISODate("2021-01-27T00:19:08.862Z")
}

We can see that the date in the second document has been wrapped with the ISODate() helper. The ISODate is in UTC.

Provide a Specific Date

You can provide your own date when calling the Date() method.

To do this, pass an ISO-8601 date string with a year within the inclusive range 0 through 9999 to the new Date() constructor or the ISODate() function. 

Here’s an example.

db.dogs.update(
  { "_id" : 1 },
  { $set : { "born" : new Date( "2020-10-07" ) } }
)

Now let’s check the document:

db.dogs.find( { "_id": 1 } ).pretty()

Result:

{
	"_id" : 1,
	"name" : "Fetch",
	"lastModified" : "Wed Jan 27 2021 10:15:53 GMT+1000 (AEST)",
	"born" : ISODate("2020-10-07T00:00:00Z")
}

We can see that the born field has been added, and it has been set to the date provided.

You can provide the date in the following formats.

Format	Description
YYYY-mm-dd	Returns the ISODate with the specified date.
YYYY-mm-ddTHH:MM:ss	Specifies the datetime in the client’s local timezone and returns the ISODate with the specified datetime in UTC.
YYYY-mm-ddTHH:MM:ssZ	Specifies the datetime in UTC and returns the ISODate with the specified datetime in UTC.
Integer	You can also provide an integer as the value. When you do this, it specifies the datetime as milliseconds since the Unix epoch (Jan 1, 1970), and returns the resulting ISODate instance.

Unix Epoch

Here’s an example of specifying an integer as the argument to new Date().

Doing this specifies the datetime as milliseconds since the Unix epoch (Jan 1, 1970), and returns the resulting ISODate instance.

db.dogs.update(
  { "_id" : 2 },
  { $set : { "born" : new Date( 1601207768012 ) } }
)

Now let’s check the document:

db.dogs.find( { "_id": 2 } ).pretty()

Result:

{
	"_id" : 2,
	"name" : "Wag",
	"lastModified" : ISODate("2021-01-27T00:19:08.862Z"),
	"born" : ISODate("2020-09-27T11:56:08.012Z")
}