Database
[{"_id":{"$oid":"5f18b5c87f9f9c0fd8322b60"},"createdAt":{"$date":"2020-07-22T21:55:20.575Z"},},{"_id":{"$oid":"5f19efac5cfa75483865eaa2"},"createdAt":{"$date":"2020-07-23T20:14:36.108Z"},}]
Query
db.collection.aggregate([{$set:{localTime:{$dateToString:{date:"$createdAt",timezone:"America/Chicago"}}}}])