var date = new Date();
var daysToDeletion = 30;
var deletionDate = new Date(date.setDate(date.getDate() - daysToDeletion));

// print the deletion date
printjson(deletionDate);

// inform how many logs will be removed
printjson(db.logs.count())
printjson(db.logs.find({timestamp : {$lt : deletionDate}}).count());

// delete old log messages
db.logs.remove({timestamp : {$lt : deletionDate}});