Date additions unreliable in Safari 2

```
Safari 2's implementation of Date#setDate uses a signed byte integer which
means large values will get 'rolled over' and return an incorrect value
unless the add/subtracts are called in increments. The following link
details this bug which affects the addition methods that use setDate. This
issue does not affect Safari 3.

http://brianary.blogspot.com/2006/03/safari-date-bug.html

I tested using the version installed at http://www.datejs.com/
```

Original issue reported on code.google.com by `cgri...@gmail.com` on 28 Nov 2007 at 2:59

Start by reproducing the reported large-date addition behavior in Safari 2 using the datejs.com installation and the linked Safari bug description. Trace the addition methods that use Date#setDate; done means large additions and subtractions return the correct date without requiring incremental calls, while Safari 3 behavior remains unaffected.

Written by the indexing model from the issue text.