Get current timestamp + offset

now.js

/**
 * @desc now("+year"), now("+3 months"), now("-13 min") or now("+86400 ms")
 */
(function () {
	var R_ADD = /(?:([+-]?)\s*(\d*))\s*(year|month|day|hour|min|sec|ms)/;

	var _getTime = Date.now || function () { return (new Date).getTime(); },
		_cache = {},
		_map = {
			'year': 'FullYear',
			'month': 'Month',
			'day': 'Date',
			'hour': 'Hours',
			'min': 'Minutes',
			'sec': 'Seconds',
			'ms': 'Milliseconds'
		}
	;


	function _add(method, x) {
		x = parseInt(x, 10) || 0;
		return function (date) {
			date['set' + method](date['get' + method]() + x);
		};
	}


	/**
	 * Get current timestamp
	 * @param   {String}  [add]
	 * @returns {Number}
	 */
	function now(add) {
		if (add) {
			var date, match;

			if (_cache[add] === void 0) {
				if (match = R_ADD.exec(add)) {
					_cache[add] = _add(_map[match[3]], match[1] + match[2]);
				} else {
					return _getTime();
				}
			}

			date = new Date;
			_cache[add](date);

			return date.getTime();
		}

		return _getTime();
	}


	// Export
	window.now = now;
})();

now("+year");
now("+3 months");
now("-13 min");
now("+86400 ms")