nice catch! didn't even notice the ```Timezone.default_dst``` option was available. I think there still might be value in allowing the ```dst``` parameter for a one-off calculation vs having to change the ```Timezone.default_dst``` temporarily, and then back again. Perhaps something like this might work:

```ruby
def ical_timezone(date, dst=Timezone.default_dst)
  period = period_for_local(date, dst)
```

This will allow the "option" to override the setting while the default (no param) will be from ```Timezone.default_dst```.

Thoughts?