为什么这个c代码的结果是"days: 33“而不是"days: 30"？

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

int main() {
    struct tm startDate = {0};
    struct tm endDate = {0};
    time_t startTime, endTime;
    int days;

    startDate.tm_year = 2021 - 1900; // 年 - 1900
    startDate.tm_mon = 0; // 月 [0-11]
    startDate.tm_mday = 1; // 日 [1-31]

    endDate.tm_year = 2021 - 1900;
    endDate.tm_mon = 1; // 二月
    endDate.tm_mday = 3; // 3号

    startTime = mktime(&startDate);
    endTime = mktime(&endDate);

    days = (endTime - startTime) / (60 * 60 * 24);

    printf("days: %d\n", days);

    return 0;
}

#include <stdio.h>

int daysInMonth(int year, int month) {
    int days[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    if (month == 1 && (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0))) {
        return 29; // 闰年2月
    }
    return days[month];
}

int main() {
    int startYear = 2021;
    int startMonth = 1;
    int startDay = 1;
    int endYear = 2021;
    int endMonth = 2;
    int endDay = 3;

    int days = 0;

    while (startYear < endYear || (startYear == endYear && startMonth < endMonth)) {
        days += daysInMonth(startYear, startMonth);
        startMonth++;
        if (startMonth > 12) {
            startMonth = 1;
            startYear++;
        }
    }

    days += endDay - startDay;

    printf("days: %d\n", days);

    return 0;
}