javascript中对日期加减的一个算法

最近在csp项目中遇到javascript中对指定日期加减一个值后获取新日期的问题，在javascipt中并没有像c#中那样强大的DateTime类（有AddDays(int)方法），所以只好写了一个函数计算

 function DayAddDiff(year, month, day, diff)...{
            var numDays = new Array(31,28,31,30,31,30,31,31,30,31,30,31);
            var isLeap = false;
            var newyear = year;
            var newmonth = month - 1;
            var n = numDays[newmonth];
            var newday = day;
            var newdiff = diff;
            var ln;
            if (newmonth == 0)
                ln = 31;
            else if(newmonth == 11)
                ln = 31;
            else
                ln = numDays[newmonth + 1];
            if (diff != 0)...{
                //判断是否润年
                if (year % 4 == 0)...{
                    if (year % 100 != 0)
                        isLeap = true;
                    else...{
                        if (year % 400 == 0)
                            isLeap = true;
                    }
                }
                if (newmonth == 1 && isLeap)
                    ++n;
                    
                if (newmonth  == 0 && isLeap) 
                    ++ln;
                //加值
                var newday = day + newdiff;
                if (newday > 0)...{
                    if (newday > n)...{
                        newday = newday - n;
                        if (newmonth == 11)
                        ...{
                            newmonth = 0;
                            newyear += 1;
                            newdiff = newday - 1;
                            return DayAddDiff(newyear, newmonth + 1, 1, newdiff);
                        }
                        else...{
                            newmonth += 1
                            newdiff = newday - 1;
                            return DayAddDiff(newyear, newmonth + 1, 1, newdiff);
                        }
                    }
                }
                else if (newday == 0)...{
                    if (newmonth == 0)
                    ...{
                        newmonth = 11
                        newyear += -1;
                        newday = 31;
                    }
                    else...{
                        newmonth += -1
                        newday = ln;
                    }
                }
                else...{
                    if (newmonth == 0)
                    ...{
                        newmonth = 11
                        newyear += -1;
                        newdiff = newday;
                        newday = 31;
                        return DayAddDiff(newyear, newmonth + 1, newday, newdiff);
                    }
                    else...{
                        newmonth += -1;
                        newdiff = newday;
                        newday = ln;
                        return DayAddDiff(newyear, newmonth + 1, newday, newdiff);
                    }
                }
            }
            //输出字符串
            var daystring = "";
            daystring += year;
            newmonth += 1;
            if (newmonth < 10)
                daystring += "/0" + newmonth;
            else
                daystring += "/" + newmonth;
            if (newday < 10)
                daystring += "/0" + newday;
            else
                daystring += "/" + newday;
            return daystring;
        }