Right, the second `w.w` could have been `null`. `dayofYearFromWeeks` does handle the null case (and also the 0 case):

```js
week = week || 1;
```

The only reason to do the parse up here is that it allows me to do addition. Otherwise, it's:

```js
> '4' + 1
'41'
```

One issue is that the first `w.w` isn't parsed, which turns out not to matter, because:

```js
> '4' - 1
3
```

which is what happens in `daysOfYearFromWeeks`. JS is strange. I think it's probably cleaner if I just parse `w.w` always, set it to `1` if it's `NaN`, and then treat it as a number from then on. I'll make that change.