documentcloud/underscore — pull request #489 by toddself (2012-02-24 22:24:55+00:00)
Title: _.isDate returns true when the date is an "Invalid Date" Date Object

Current behavior

```javascript
> var test_date = new Date('today');
> test_date
Invalid Date
> _.isDate(test_date)
true
```

New behavior

```javascript
> var test_date = new Date('today')
> test_date
Invalid Date
> _.isDate(test_date)
false
```

While `test_date` above is [object Date}, it's not useful since `test_date.valueOf()`, `.getMonth()`, `.getDay()`, etc all return `NaN`.