code: function(y, m) {
              var num = 31,isLeap = (y % 100 !== 0 && y % 4 === 0) || (y % 400 === 0);
              switch (parseInt(m)) {
                  case 2: num = isLeap ? 29 : 28; break;
                  case 4: case 6: case 9: case 11: num = 30; break;
              }
              return num;
          }
docs: 获取本月的总天数