Date times appear to be slightly more permissive than RFC 3339

libtoml accepts date time literals where the month or day of the month are zero - I'm not super familiar with RFC 3339 but my reading of it is that this isn't valid.

Example input:

```
a=2000-00-00T00:00:00Z
```

Output:

```
a = 1999-11-30T00:00:00Z
```
