MongoDB provides the NOW system variable that allows you to get the current datetime value when using an aggregation pipeline.

This can be useful for when you want to update a document with the current datetime.

Starting in MongoDB 4.2, update methods can can accept an aggregation pipeline. Therefore, NOW can be used as an alternative to the $currentDate field update operator for setting the current datetime when using any of the update methods.

To access the NOW system variable, prefix it with two dollar signs and surround it in quotes ("$$NOW").

Example

Suppose we have a collection called dogs that contains the following document:

{ "_id" : 1, "name" : "Fetch", "weight" : 25 }

Here’s an example of using the NOW system variable when updating that document:

db.dogs.updateOne(
  { _id : 1 },
  [
    { $set : { weight : 30, lastModified : "$$NOW" } }
  ]
)

Now let’s take a look at the document again:

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

Result:

{
	"_id" : 1,
	"name" : "Fetch",
	"weight" : 30,
	"lastModified" : ISODate("2021-01-27T01:29:32.833Z")
}

The lastModified field has been added with a Date object that contains the current date and time.

There’s also a CLUSTER_TIME system variable that returns the current timestamp, although this is only available on replica sets and sharded clusters.

Also note that the NOW and CLUSTER_TIME values remain the same throughout the pipeline.