phpjs/experimental/datetime/timezone_offset_get.js

php.js offers community built php functions in javascript

// Note: basically implemented, but useless without DateTimeZone object
function timezone_offset_get(dtzObj, datetime) {
  // http://kevin.vanzonneveld.net
  // +   original by: Brett Zamir (http://brett-zamir.me)

  return dtzObj.getOffset(datetime);
}