Count of records by Date MongoDB

if you want to count by lastModified date and time then can use just like:

db.getCollection('collectionName').aggregate({$group:{_id: "$lastModified", count:{$sum:1}}})

and if you want to count by lastModified date only then can use just like:

db.getCollection('collectionName').aggregate(
[
    {
        $group:
        {
            _id:
            {
                day: { $dayOfMonth: "$lastModified" },
                month: { $month: "$lastModified" }, 
                year: { $year: "$lastModified" }
            }, 
            count: { $sum:1 },
            date: { $first: "$lastModified" }
        }
    },
    {
        $project:
        {
            date:
            {
                $dateToString: { format: "%Y-%m-%d", date: "$date" }
            },
            count: 1,
            _id: 0
        }
    }
])

This is simpler:

db.getCollection('collectionName').aggregate(
[
    {
        $group:
        {
            _id: {
                $dateToString: {
                    "date": "$lastModified",
                    "format": "%Y-%m-%d"
                }
            }, 
            count: { $sum:1 }
        }
    }
])