Database
[{info:{startDate:ISODate("2020-01-01T19:15:15Z")}},{info:{startDate:ISODate("2020-01-05T00:00:00Z")}}]
Query
db.collection.update({},[{$set:{"info.startDate":{$cond:[{$ne:[{$hour:"$info.startDate"},0]},{$dateFromParts:{"year":{$year:{$add:["$info.startDate",86400000]}},"month":{$month:{$add:["$info.startDate",86400000]}},"day":{$dayOfMonth:{$add:["$info.startDate",86400000]}},"hour":0,"minute":0,"second":0,"millisecond":0,}},"$info.startDate"]}}}],{"multi":true,"upsert":false})