How to define default date values in Symfony2 routes?

How to define default date values in Symfony2 routes?

If I want to create a route, where the year, month and date are variables,
how can I define that if these variables are empty, the current date shall
be taken?
E.g. like this (doesn't work for sure...)
blog:
path: /blog/{year}/{month}/{day}
defaults: { _controller: AcmeBlogBundle:Blog:index,
year: current_year,
month: current_month
day: current_day
}
I thought about defining two different routes, like this
blog_current_day:
path: /blog
defaults: { _controller: AcmeBlogBundle:Blog:index }
blog:
path: /blog/{year}/{month}/{day}
defaults: { _controller: AcmeBlogBundle:Blog:index }
But if I then call blog_current_day my controller
public function indexAction(Request $request, $year, $month, $day) {
// ...
}
will throw an exception because year, month and day are missing.
Any suggestions?