[RESOLVED] AddDays: 1970-1-1 minus 7 days

[DaveDavis]

Refer to AddDays procedure.
void AddDays(int& year, int& month, int& day, int numDays)

int year = 1970;
int month = 1;
int day = 8;
int numDays = -7;
AddDays(year,month,day, numDays);

The year, month and day returns: 1900, 0, -1

int year = 1900;
int month = 1;
int day = 1;
int numDays = 7;
AddDays(year,month,day, numDays);

The year, month and day returns: 1970, 1, 8



The identified problem with the code is related to handling dates before the UNIX epoch (1st January 1970). How to correct it?

void CCalendar::AddDays(int& year, int& month, int& day, int numDays) 
{    
    // Convert year, month, and day to time_t
    struct tm date = {0};
    date.tm_year = year - 1900;  // Years since 1900
    date.tm_mon = month - 1;     // Months since January (0-11)
    date.tm_mday = day;          // Day of the month (1-31)    
    
	// Convert to time_t
    time_t time = mktime(&date);
    // Add or subtract numDays
    time += numDays * 24 * 60 * 60; // Convert days to seconds

	struct tm newDate;
    localtime_s(&newDate, &time);
    year = newDate.tm_year + 1900;
    month = newDate.tm_mon + 1;
    day = newDate.tm_mday;
    
    // Ensure the resulting date is within the valid range
    /*if (year < 1900) {
        year = 1900;
        month = 1;
        day = 1;
    } else if (year > 9999) {
        year = 9999;
        month = 12;
        day = 31;
    }*/
}

[DaveDavis]

OK, I abandon c++'s mktime and use hard code.

AddDays(int& year, int& month, int& day, int numDays) 
{        
	 while (numDays != 0) 
	 {
             // Get the number of days in the current month
            int daysInCurrentMonth = GetDaysInMonth(year, month);
        
            if (numDays > 0) 
	   {
                // Adding days
                if (day + numDays <= daysInCurrentMonth) 
		{
                     day += numDays;
                     numDays = 0;
                 } 
		else 
		{
                    numDays -= (daysInCurrentMonth - day + 1);
                    day = 1;
                    month++;
                
                    if (month > 12) 
		   {
                        month = 1;
                        year++;
                    }
               }
           } 
	  else 
	  {
                  // Convert negative numDays to positive
		  numDays = -numDays;
		  while (numDays > 0) 
		  {
			if (numDays >= day) 
			{
				numDays -= day;
				month--;
				if (month < 1) 
				{
					month = 12;
					year--;
				}
				day = GetDaysInMonth(year, month);
			} 
			else 
			{
				day -= numDays;
				numDays = 0;
			}
		}
	}
    }

    // Ensure the resulting date is within the valid range
    if (year < 1900) {
        year = 1900;
        month = 1;
        day = 1;
    } else if (year > 9999) {
        year = 9999;
        month = 12;
        day = 31;
    }
}