Why generate an ObjectId from a timestamp?

To query documents by creation date.

e.g. to find all comments created after 2020-08-05 00:00:00 UTC+7:

db.comments.find({_id: {$gt: ObjectId("5f2994100000000000000000")}})

Javascript functions

var objectIdFromDate = function (date) {
    return Math.floor(date.getTime() / 1000).toString(16) + "0000000000000000";
};

var dateFromObjectId = function (objectId) {
    return new Date(parseInt(objectId.substring(0, 8), 16) * 1000);
};