Fixing the Date Class increment (C++) - c++

Question

I have made a Library system. But I just checked and my 'Date' is not working properly.
The primary task of this class
(1) Increment date
Now, I am having an issue in the increment date, it works fine when I am incrementing it to the values up to 25-30. But when I enter 90, it messes up.
void increment_date(int num)
{
int day;
int month_new;
setDay(getDay()+num);
if( getDay()>Days_per_Month[getMonth()] )
{
day=getDay()-Days_per_Month[getMonth()];
setDay(day);
setMonth(getMonth()+1);
if(Days_per_Month[getMonth()]>12)
{
month_new=1;
setMonth(month_new);
setYear(getYear()+1);
}
}
cout<<"Return Date: ";
Print_Date();
}
//the code below is outside the class.
const int Date:: Days_per_Month[13]={0,31,28,31,30,31,30,31, 31, 30, 31, 30, 31};
int Date::checkDay(int testday) //returntype classname :: funcname (parameteres)
{
//static const int Days_per_Month[13]={0,31,28,31,30,31,30,31, 31, 30, 31, 30, 31};
if(testday > 0 && testday <= Days_per_Month[Month])
return testday;
if ( Month==2 && testday==29 && (Year%400==0 || (Year%4==0 && Year%100!=0)) ) //for leap year
return testday;
cout<<"Day "<<testday<<" invalid. Set to day 1."<<endl;
return 1;
}
enter image description here

Answer 1

setDay(getDay()+num);
if( getDay()>Days_per_Month[getMonth()] )
{
day=getDay()-Days_per_Month[getMonth()];
setDay(day);
setMonth(getMonth()+1);
In the above code, you add num to the value returned by getDay(), and then you check to see if getDay() is greater than the number of days in the current month, and if it is, you try to correct the problem by subtracting the number of days in the current month, and then incrementing the month.
So far, so good, but what if getDay() is still greater than the number of days in the current month after you've done that subtraction? (e.g. what if the user enters 300 for the increment-count?) In that case, you need to do the whole thing again, and keep doing it until getDay() is small enough to be a valid day-within-the-month value. So really a while loop is called for, not just an if.