How does mongodb backup to support recovery to any point in time

  mongodb, question

I currently use mongodump –oplog to back up the database, but I don’t know how to restore to the specified point in time.
Even if this “point in time” is based on “days”, it is better than not

In addition, there is an oplog.bson in the file that I have backed up, and there is nothing in it. is this normal?

mongodump --host yourdbhost --port 27017 --db dbname --collection collectionname
 -o mongodb --query '{"relativeStamp":{$lt:1389949842}}'

Do you want this backup?