Get the last date of month in YYYYMMDD format in C++

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C++

Windows

Currently in my code i will get some date from the file reading like 'CString tsDate' in the format of YYYYMMDD(ex: 20180501)
and i need to check whether 'tsDate' is last day of that month. It means for MAY month it is 20180531 so i nned to get this date.

In my code currently am using COleDateTime class to get GetMonth() and GetYear.

Can anyone suggests me to get the last date of the month?

What I have tried:

I have tried the below :
int getNumberOfDays(int month, int year)
{
//leap year condition, if month is 2
if( month == 2)
{
if((year%400==0) || (year%4==0 && year%100!=0))
return 29;
else
return 28;
}
//months which has 31 days
else if(month == 1 || month == 3 || month == 5 || month == 7 || month == 8
||month == 10 || month==12)
return 31;
else
return 30;
}

But here months are hardcoded but every year number of days may vary right?