How to calculate the number of days between two given dates? (Leap year obstacle)

Question

• Any year evenly divisible by 400 is a leap year (e.g., 2000 was a leap year).
• Any other year evenly divisible by 100 is not a leap year (e.g., 1700, 1800 and

Answer 1

#include 
#define SECONDS_PER_DAY (24 * 60 * 60)

time_t time_from_date(int year, unsigned month, unsigned day)
{
    return mktime(&(struct tm){
        .tm_year = year - 1900, .tm_mon = month - 1, .tm_mday = day });
}

int days_between(int year0, unsigned month0, unsigned day0,
    int year1, unsigned month1, unsigned day1)
{
    return difftime(time_from_date(year1, month1, day1),
        time_from_date(year0, month0, day0)) / SECONDS_PER_DAY;
}