获取月最大天数

获取某月最大天数
参数：y = year，m = month
返回：len = 31/30/29/28

function(y, m) {
    var len = m == 1 || m == 3 || m == 5 || m == 7 || m == 8 || m == 10 || m == 12 ? 31 : m == 4 || m == 6 || m == 9 || m == 11 ? 30 : y % 4 == 0 && y % 100 != 0 ? 29 : y % 400 == 0 ? 29 : 28
    return len
  }

function(y, m) {
    var len =0;
     if (m == 1 || m == 3 || m == 5 || m == 7 || m == 8 || m == 10 || m == 12) {
       len = 31;
     } else if (m == 4 || m == 6 || m == 9 || m == 11) {
       len = 30;
     } else {
       if (y % 4 == 0 && y % 100 != 0) {
         len = 29;
       } else if (y % 400 == 0) {
         len = 29;
       } else {
         len = 28;
       }
     }
    return len
  }