[{_id:"123",date:"1900-01-01T11:00:00.0000000",name:"joe",birthday:"1999-01-01"}]

db.collection.aggregate([{"$addFields":{"date":{"$toDate":"$date"},"birthday":{"$toDate":"$birthday"}}},{"$merge":{"into":"collection2","on":"_id","whenMatched":"merge","whenNotMatched":"insert"}}])