Петр Петров

var data = [
           '08-10-2011',
           '08-04-2007',
           '03-11-2008',
           '03-03-2008',
           '03-14-2008',

           '03-28-2005',
           '03-20-2005'
       ];

       function decodeDate(arr) {
           var i, len = data.length,
                   obj = {},
                   date,
                   year,
                   month;

           for (i = 0; i < len; i++) {
               date=arr[i].split("-");
               if(!obj[year=date[2]])obj[year]={};
               if(!obj[year][month=date[0]]){
                   obj[year][month]=[date[1]];
               }else{
                   obj[year][month].push(date[1]);
               }
           }
           return obj;
       }

       console.log(decodeDate(data));