phpjs/experimental/datetime/date_add.js

php.js offers community built php functions in javascript

function date_add(dt, interval) {
  // http://kevin.vanzonneveld.net
  // +   original by: Brett Zamir (http://brett-zamir.me)
  // *     example 1: var dt = date_create();
  // *     example 1: var dti = date_interval_create_from_date_string('+1 day');
  // *     example 1: date_add(dt, dti);
  // *     returns 1: {}

  // Fix: need to check argument to the dti example above

  return dt.add(interval);
}