how to use “$dateToString” into mongo db 2.6 is any substitute?

I made a script in local development machine mongoDb 4.0 its working flawless but for client server MongoDb is 2.6 so $dateToString is not supported. is there any substitute using $dateToString into mongoDb 2.6.

Using MongoDb 4.0 :

db.getCollection('orgData').aggregate([
    {$match:{'orgId' : 5} },
    {$unwind :'$events.click'},
    {'$project' :{'events.click' : 1}},
    {$group :{
            '_id' :{'$dateToString' : {format: "%Y-%m-%d",date:'$events.click.mongo_datetime'} }
            ,'count' : {'$sum' : 1}
        }}
        ]);

Output :

/* 1 */
{
    "_id" : "2019-03-01",
    "count" : 1427.0
}

/* 2 */
{
    "_id" : "2019-02-28",
    "count" : 2244.0
}

But in MongoDb 2.6 Iam getting Error :

assert: command failed: {
        "errmsg" : "exception: invalid operator '$dateToString'",
        "code" : 15999,
        "ok" : 0
} : aggregate failed
Error: command failed: {
        "errmsg" : "exception: invalid operator '$dateToString'",
        "code" : 15999,
        "ok" : 0
} : aggregate failed
    at Error (<anonymous>)
    at doassert (src/mongo/shell/assert.js:11:14)
    at Function.assert.commandWorked (src/mongo/shell/assert.js:244:5)
    at DBCollection.aggregate (src/mongo/shell/collection.js:1149:12)
    at (shell):1:29
2019-09-13T17:28:57.980+1000 Error: command failed: {
        "errmsg" : "exception: invalid operator '$dateToString'",
        "code" : 15999,
        "ok" : 0
} : aggregate failed at src/mongo/shell/assert.js:13

Database design :

Answer

In MongoDB 2.6 you would need to use the date operators together with the string operator $concat to achieve the desired expression as substitute. Consider the following example :

db.getCollection('orgData').aggregate([
    { '$match': { 'orgId' : 5 } },
    { '$unwind': '$events.click' },
    { '$group' :{
        '_id': {
            '$concat': [
                { '$substr': [
                    { '$year': '$events.click.mongo_datetime' },
                    0,4
                ] },
                '-',
                { '$substr': [
                    { '$month': '$events.click.mongo_datetime' },
                    0,2
                ] },
                '-',
                { '$substr': [
                    { '$dayOfMonth': '$events.click.mongo_datetime' },
                    0,2
                ] }
            ]
        },
        'count': { '$sum' : 1 }
    } }
]);

Note: the $project pipeline stage becomes redundant if used before a $group pipeline step since $group will change the document schema as a result.