i've added a date filter (node: post date), with granularity year to the page display.
operator : equal to 2011.
the default display (when no month is given) it is ok. i can switch the years.( the url argument is date_filter[value][year]=2011)

but when i click on a month it says : "An illegal choice HAS BEEN DETECTED. Please contact the site administrator." and gives no result. the url argument is now : date_filter[value]=2011-01-01%2000%3A00%3A00&date_filter[default_date]=&date_filter[default_to_date]=

if i manualy change the argument into date_filter[value][year]=2011&date_filter[default_date]=&date_filter[default_to_date]= it's working.