Group by dates in MongoDB?

You can use the aggregate framework to group by dates in MongoDB. The aggregation pipeline provides powerful date operators to group documents by specific date components like day, month, or year.

Sample Data

Let us first create a collection with some documents containing date fields ?

db.groupByDateDemo.insertMany([
    {"UserLoginDateTime": new ISODate()},
    {"UserLoginDateTime": new ISODate("2019-01-31T15:20:09.234Z")},
    {"UserLoginDateTime": new ISODate("2017-04-21T16:12:13.240Z")},
    {"UserLoginDateTime": new ISODate("2016-05-25T19:11:21.130Z")},
    {"UserLoginDateTime": new ISODate("2016-05-25T19:11:21.130Z")},
    {"UserLoginDateTime": new ISODate()}
]);

Display all documents from the collection using the find() method ?

db.groupByDateDemo.find().pretty();

{
    "_id" : ObjectId("5c6ee4df6fd07954a4890695"),
    "UserLoginDateTime" : ISODate("2019-02-21T17:50:23.076Z")
}
{
    "_id" : ObjectId("5c6ee51c6fd07954a4890696"),
    "UserLoginDateTime" : ISODate("2019-01-31T15:20:09.234Z")
}
{
    "_id" : ObjectId("5c6ee5336fd07954a4890697"),
    "UserLoginDateTime" : ISODate("2017-04-21T16:12:13.240Z")
}
{
    "_id" : ObjectId("5c6ee54b6fd07954a4890698"),
    "UserLoginDateTime" : ISODate("2016-05-25T19:11:21.130Z")
}
{
    "_id" : ObjectId("5c6ee8de6fd07954a4890699"),
    "UserLoginDateTime" : ISODate("2016-05-25T19:11:21.130Z")
}
{
    "_id" : ObjectId("5c6ee8e76fd07954a489069a"),
    "UserLoginDateTime" : ISODate("2019-02-21T18:07:35.208Z")
}

Group by Dates Using Aggregation

Here is the query to group by dates in MongoDB using the aggregation pipeline ?

db.groupByDateDemo.aggregate([
    {
        $group: {
            _id: {
                $add: [
                    { $dayOfYear: "$UserLoginDateTime"},
                    { $multiply: [400, {$year: "$UserLoginDateTime"}] }
                ]
            },
            NumberOfTimes: { $sum: 1 },
            f: {$min: "$UserLoginDateTime"}
        }
    },
    { $sort: {_id: 1} },
    { $limit: 10 },
    { $project: { date: "$f", NumberOfTimes: 1, _id: 0} }
]);

{ "NumberOfTimes" : 2, "date" : ISODate("2016-05-25T19:11:21.130Z") }
{ "NumberOfTimes" : 1, "date" : ISODate("2017-04-21T16:12:13.240Z") }
{ "NumberOfTimes" : 1, "date" : ISODate("2019-01-31T15:20:09.234Z") }
{ "NumberOfTimes" : 2, "date" : ISODate("2019-02-21T17:50:23.076Z") }

How It Works

The aggregation pipeline uses several date operators:

• $dayOfYear extracts the day number within the year (1-366)
• $year extracts the year component from the date
• $multiply and $add create a unique identifier for each date
• $sum: 1 counts occurrences for each grouped date
• $min selects the earliest timestamp for that date

Conclusion

MongoDB's aggregation framework provides powerful date grouping capabilities using operators like $dayOfYear and $year. This approach effectively counts document occurrences per date and can be extended for monthly or yearly grouping patterns.