dynamically changing default parameter?

I am creating SSRS reports on top of SSAS cubes. I want the default value of parameter to change dynamically based on the current year or it should select the last of the parameter values.

Can this be done?

here is how i'm setting my "from date" (datetime) param to be the first of the current month
=DateSerial(Year(Now()), Month(Now()),1)

for today, just use
=Today()

this is for a whole date, you can just pick the year part