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#include <time.h>
#include <stdbool.h>
#include <stdio.h>
bool leapyear(unsigned year) {
/* All years that are divisible by 4 are leap years,
unless they start a new century, provided they
are not divisible by 400. */
return !(year % 4) && ((year % 100) || !(year % 400));
}
#define DAYS_BEFORE \
(int const[12]){ \
[0] = 0, [1] = 31, [2] = 59, [3] = 90, \
[4] = 120, [5] = 151, [6] = 181, [7] = 212, \
[8] = 243, [9] = 273, [10] = 304, [11] = 334, \
}
struct tm time_set_yday(struct tm t) {
// tm_mdays starts at 1.
t.tm_yday += DAYS_BEFORE[t.tm_mon] + t.tm_mday - 1;
// Takes care of leap years
if ((t.tm_mon > 1) && leapyear(t.tm_year+1900))
++t.tm_yday;
return t;
}
int main(void) {
struct tm today = {
.tm_year = 2019-1900,
.tm_mon = 4-1,
.tm_mday = 3,
.tm_hour = 10,
.tm_min = 0,
.tm_sec = 47,
};
printf("this year is %d, next year will be %d\n",
today.tm_year+1900, today.tm_year+1900+1);
today = time_set_yday(today);
printf("day of the year is %d\n", today.tm_yday);
}
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