lib/interval.js
function add(base, addend) {
  if (util.isDate(base)) {
    return new Date(base.getTime() + interval(addend));
  }
  return interval(base) + interval(addend);
}
function add(base, addend) {
  if (util.isDate(base)) {
    return new Date(base.getTime() + interval(addend));
  }
  return interval(base) + interval(addend);
}
function add ( base , addend ) { if ( util . isDate ( base ) ) { return new Date ( base . getTime ( ) + interval ( addend ) ) ; } return interval ( base ) + interval ( addend ) ; }
first parmater can be a date or an interval, second parameter has to be an interval returns the same type as the first parameter
first parmater can be a date or an interval second parameter has to be an interval returns the same type as the first parameter