Converting string to date in mongodb

Using MongoDB 4.0 and newer

The $toDate operator will convert the value to a date. If the value cannot be converted to a date, $toDate errors. If the value is null or missing, $toDate returns null:

You can use it within an aggregate pipeline as follows:

db.collection.aggregate([
    { "$addFields": {
        "created_at": {
            "$toDate": "$created_at"
        }
    } }
])

The above is equivalent to using the $convert operator as follows:

db.collection.aggregate([
    { "$addFields": {
        "created_at": { 
            "$convert": { 
                "input": "$created_at", 
                "to": "date" 
            } 
        }
    } }
])

---

Using MongoDB 3.6 and newer

You cab also use the $dateFromString operator which converts the date/time string to a date object and has options for specifying the date format as well as the timezone:

db.collection.aggregate([
    { "$addFields": {
        "created_at": { 
            "$dateFromString": { 
                "dateString": "$created_at",
                "format": "%m-%d-%Y" /* <-- option available only in version 4.0. and newer */
            } 
        }
    } }
])

---

Using MongoDB versions >= 2.6 and < 3.2

If MongoDB version does not have the native operators that do the conversion, you would need to manually iterate the cursor returned by the find() method by either using the forEach() method
or the cursor method next() to access the documents. Withing the loop, convert the field to an ISODate object and then update the field using the $set operator, as in the following example where the field is called created_at and currently holds the date in string format:

var cursor = db.collection.find({"created_at": {"$exists": true, "$type": 2 }}); 
while (cursor.hasNext()) { 
    var doc = cursor.next(); 
    db.collection.update(
        {"_id" : doc._id}, 
        {"$set" : {"created_at" : new ISODate(doc.created_at)}}
    ) 
};

For improved performance especially when dealing with large collections, take advantage of using the Bulk API for bulk updates as you will be sending the operations to the server in batches of say 1000 which gives you a better performance as you are not sending every request to the server, just once in every 1000 requests.

The following demonstrates this approach, the first example uses the Bulk API available in MongoDB versions >= 2.6 and < 3.2. It updates all
the documents in the collection by changing the created_at fields to date fields:

var bulk = db.collection.initializeUnorderedBulkOp(),
    counter = 0;

db.collection.find({"created_at": {"$exists": true, "$type": 2 }}).forEach(function (doc) {
    var newDate = new ISODate(doc.created_at);
    bulk.find({ "_id": doc._id }).updateOne({ 
        "$set": { "created_at": newDate}
    });

    counter++;
    if (counter % 1000 == 0) {
        bulk.execute(); // Execute per 1000 operations and re-initialize every 1000 update statements
        bulk = db.collection.initializeUnorderedBulkOp();
    }
})
// Clean up remaining operations in queue
if (counter % 1000 != 0) { bulk.execute(); }

---

Using MongoDB 3.2

The next example applies to the new MongoDB version 3.2 which has since deprecated the Bulk API and provided a newer set of apis using bulkWrite():

var bulkOps = [],
    cursor = db.collection.find({"created_at": {"$exists": true, "$type": 2 }});

cursor.forEach(function (doc) { 
    var newDate = new ISODate(doc.created_at);
    bulkOps.push(         
        { 
            "updateOne": { 
                "filter": { "_id": doc._id } ,              
                "update": { "$set": { "created_at": newDate } } 
            }         
        }           
    );

    if (bulkOps.length === 500) {
        db.collection.bulkWrite(bulkOps);
        bulkOps = [];
    }     
});

if (bulkOps.length > 0) db.collection.bulkWrite(bulkOps);