Fixed the year 2038 problem.

There were some integer overflow. So it caused problems like:
```javascript
var d = new time.Date('2038-1-19 03:14:07 AM', 'UTC');
console.log( d.toString());
console.log( d.getTime());
```
Result:
```
Sat Dec 14 1901 03:14:07 GMT+0000 (UTC)
-2147460353000
```