MongoDB $dateFromString error in aggregation

I am facing this error while using $dateFromString

MongoServerError: PlanExecutor error during aggregation :: caused by :: $dateFromString requires that 'dateString' be a string, found: date with value 2022-09-08T12:45:30.811Z

In my schema, I did { timestamps: true } to save createdAt and updatedAt.

db.collection.aggregate([{
  $group: {
    _id: {
      createdAt: {
        $dateFromString: {
          dateString: '$createdAt'
        }
      }
    }
  }
}])

The object in mongodb is saved as:

{
  _id: new ObjectId("6319e3ea8fec56ded0173275"),
  isAvailable: true,
  createdAt: 2022-09-08T12:45:30.811Z,
  updatedAt: 2022-09-08T12:45:30.811Z,
  __v: 0
}