How do i correctly add days to given date? - c++

Question

I made this operator for class where I store date, it is supposed to add given number of days to date and convert it back to human format
CDate CDate::operator+(const int x)
{
time_t t;
struct tm tmp = {0};
struct tm newtime = {0};
tmp.tm_year = timeinfo.tm_year;
tmp.tm_mon = timeinfo.tm_mon;
tmp.tm_mday = timeinfo.tm_mday + x;
t = mktime(&tmp);
newtime = *(localtime(&t));
return CDate(newtime.tm_year,newtime.tm_mon,newtime.tm_mday);
}
problem is it compute date 3 days higher that it should
concrete problem is, I have date 2000-01-02 (year-month-day) and i try to add 1500 days, expected result is 2004-02-10 but I am getting 2004-02-13
check
what am I doing wrong here?
my code:
http://pastebin.com/wLxzTe3W

Answer 1

You have two errors.
First, the tm passed to mktime is invalid: The year shouldn´t be the absolute year like 2000, but the year difference from 1900, ie. 100 for 2000. With this problem corrected, I get 2004-02-12 as result (neither 2004-02-13 nor 2004-02-10).
(On my computer, the program (without any changes) just crashes: mktime returns -1, localtime can´t handle it and returns an invalid pointer, which is used in the same line...)
Second, in the same struct, the day may be 1-31, but the month only 0-11 (not 1-12). Your 2000-01-02 is the second day of February, not January.
Knowing this, the result is correct (I´ll write the full year, but month starting from 0):
1500 days to add to 2000-01-02:
2000 has 366 days (leap year), and we don´t need January and the first February day:
2000-01-02 + (366-31-1) days = 2001-00-01, 1166 days to add left
Another 365 days makes 2002-00-01, 801 days to add left
Another 365 days makes 2003-00-01, 436 days to add left
Another 365 days makes 2004-00-01, 71 days to add left
Another 31 of January makes 2004-01-01, 40 days to add left
Another 29 of leap February makes 2004-02-01, 11 days to add left
Another 11 added makes 2004-02-12
Read http://www.cplusplus.com/reference/ctime/tm/ carefully

Answer 2

This is the algorithm for adding days to the date.
Calculate day number from date.
function g(y,m,d)
m = (m + 9) % 12
y = y - m/10
return 365*y + y/4 - y/100 + y/400 + (m*306 + 5)/10 + ( d - 1 )
To add no. of days in the date->
g(y2,m2,d2) + no_of_days
After this, you can convert your days back to date.
function d(g)
y = (10000*g + 14780)/3652425
ddd = g - (365*y + y/4 - y/100 + y/400)
if (ddd < 0) then
y = y - 1
ddd = g - (365*y + y/4 - y/100 + y/400)
endif
mi = (100*ddd + 52)/3060
mm = (mi + 2)%12 + 1
y = y + (mi + 2)/12
dd = ddd - (mi*306 + 5)/10 + 1
return y, mm, dd