You should rather use the DateTimeFormat object for that:

```javascript
    var dateFormat = _('graph-date-format') || '%b %e';
    var f = new navigator.mozL10n.DateTimeFormat();
    var todayTag = f.localeFormat(d, model.axis.X.today, dateFormat).toUpperCase();
```

That ensures the month name will be properly localized.
Oh, and I’d suggest using %e rather than %d to avoid the leading zero, but that’s just my two cents.