add clone, add for javascript Date object

date_extension.js

Date.prototype.clone = function () {
  return new Date(this.getTime());
};

Date.prototype.add = function (val, unit) {
  d = new Date(this.getTime());
  switch (unit) {
    case 'year':
    case 'years':
      d.setYear(d.getYear() + val);
      break;
    case 'month':
    case 'months':
      d.setMonth(d.getMonth() + val);
      break;
    case 'day':
    case 'days':
      d.setDate(d.getDate() + val);
      break;
    case 'hour':
    case 'hours':
      d.setHours(d.getHours() + val);
      break;
    case 'minute':
    case 'minutes':
      d.getMinutes(d.getMinutes() + val);
      break;
    case 'second':
    case 'seconds':
      d.getSeconds(d.getSeconds() + val);
      break;
    default:
      throw 'Invalid Unit; should be years, month, days, hours, minutes, seconds';
  }
  return d;
};