Mongo Date ObjectId helpers

gistfile1.js

var daysAgo = 10;

Date.prototype.toObjectId = function() {
  var dateSecs = Math.floor(this.getTime() / 1000);
  return ObjectId(dateSecs.toString(16) + "0000000000000000");
}

function daysAgoToDate(daysAgo) {
  return new Date(new Date().setDate(new Date().getDate() - daysAgo));
}

var objectIdAgo = daysAgoToDate(daysAgo).toObjectId();