Dartで日付の計算をする ref: http://qiita.com/items/2612d3f6114be93e54b2

date-calculator.dart

void main() {
  var start_day = new Date(2012,8,3,0,0,0,0);

  var one_week_ago = new Date(2012,8,10,0,0,0,0);

  var diff = one_week_ago.difference(start_day);

  print(diff.inDays);//7
  var difference =  difference(start_day, one_week_ago);
  print(difference);//5
}

int difference(start,end){
  var diff = 0;
  while(start < end){
    // start の曜日を調べる
    var weekday = start.weekday;
    // 土曜、日曜でないならdiffを増やす
    if(weekday !== Date.SAT && weekday !== Date.SUN){
      diff++;
    }
    // startを1日進める
    start = start.add(const Duration(1, 0, 0, 0));
  }
  return diff;
}

今のバージョンにDateはありますか？