phpjs/experimental/datetime/date_modify.js

php.js offers community built php functions in javascript

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

  return dt.modify(modify);
}