How to count number of days between two dates using <time.h>

Question

I'm trying to code ac function that calculates de number of DAYS between two dates ( format: yyyy-mm-dd ) using time.h.

The date is read from a string. I don't need to account for seconds or minutes.

I know I probably need to use difftime() but how can It be used in that format?

Thank you,

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This is what I tried to do but I get segmentation fault.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

#define CURRDATE "2019-10-5"
#define OTHER "2019-10-10"

void auxDate(struct tm *t, char *date)
{
    struct tm p;

    p.tm_year = atoi(strtok(date, "-")) - 1900;
    p.tm_mon = atoi(strtok(date, "-"));
    p.tm_mday = atoi(date);

    *t = p;
}
int main()
{
    struct tm date1, date2;

    auxDate(&date1, CURRDATE);
    auxDate(&date2, OTHER);

    printf("%.0lf\n", difftime(mktime(&date1), mktime(&date2)));
}

Answer 1

Code fails for various reasons.

"2019-10-5" is a string literal that strtok(date, "-") attempts to modify. This leads to undefined behavior (UB). Certainly the cause of the segmentation fault.

Instead use void auxDate(struct tm *t, const char *date) and parse date without attempting to change it. See strspn() and strcspn() .

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mktime(&date1) uses a not completely assigned struct tm . Initialize all struct tm members with struct tm p = { 0 }; . Otherwise the uninitialized members can provide erroneous values to mktime() .

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.tm_mon is the number of month since January. I'd expect a - 1 somewhere.

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Since difftime() return the difference in seconds and the goal is difference in days. Division by the seconds-per-day is missing.

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The are other corner case to solve that involve daylight savings time and redefinitions of a timezone's offset that will shift the time differences by a hour or so.

To handle, code could use the time at noon and set the DST to -1

p.tm_hour = 12;
p.tm_isdst = -1;

Then round the division:

days = round(difftime(...)/(24.0*60*60));

Even this will not handle very rare cases when a Pacific area timezone changed which side of the international date line it was on, which effectively repeated or cancelled a day.

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On the good side, your posted code does compile.