Database
[{"_id":ObjectId("5b36c7fdd515e80009e7cc84"),"code":"1","file":{"dates":{"start":ISODate("2018-06-11T22:00:00.000Z"),"end":ISODate("2018-06-11T22:00:00.000Z")}}},{"_id":ObjectId("5b36c7ffd515e80009e7cf03"),"code":"15","file":{"dates":{"start":ISODate("2018-09-02T22:00:00.000Z"),"end":ISODate("2019-09-26T22:00:00.000Z")}}}]
Query
db.collection.update({},[{"$set":{"file.dates.start":{$add:["$file.dates.start",7200000]},"file.dates.end":{$add:["$file.dates.end",7200000]}}}],{multi:true})