Database
[{"paymentDate":new Date("2021-11-19T05:36:32.596+00:00")},{"paymentDate":"2021-11-19T05:36:32.596+00:00"}]
Query
db.collection.find({},{convertedDate:{$cond:{if:{"$eq":[{$type:"$paymentDate"},"date"]},then:"$paymentDate",else:{$dateFromString:{dateString:"$paymentDate"}}}}})