Database
[{"_id":1,"time_started":"2021-11-20T00:00:00.000Z"}]
Query
db.collection.update({"_id":1},[{"$set":{"time_completed":"2021-11-21T00:00:00.000Z","duration":{"$subtract":[ISODate("2021-11-21T00:00:00Z"),{"$dateFromString":{"dateString":"$time_started"}}]}}}])