Getting the elapsed number of days

Hello,

Code:

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gcc 4.7.2
c89

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I have developed a sample program that should get the difference between a start time and an end time. I then use the seconds to calculate how many days has elapsed between those 2 times.

This is for a logging application that after a user specified number of days. The logs will be rolled over and a new start time will be set. The user can decide how many days the log will be replaced.

I added a small sleep to get the general idea, but normally this will be logging for days.

Is this the best and most portable (linux/windows) way to do this? Any potential pitfalls in this design?

Many thanks for any suggestions,

Code:

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

int main(void)
{
    time_t start_timestamp;
    time_t end_timestamp;
    struct tm time_days;
    double seconds = 0;
    size_t cumulative_days = 0;
#define FORMAT_TIME_SIZE 64
    char format_time[FORMAT_TIME_SIZE];

#define DAY 3600    /* Seconds in one day */
#define MAX_DAYS 10  /* Roll over log after 10 days */

    /* Current time of starting application */
    time(&start_timestamp);
    time_days = *localtime(&start_timestamp);

    /* Print the current time at the start */
    memset(format_time, 0, sizeof format_time);
    strftime(format_time, sizeof format_time, "%c", &time_days);
    printf("timestamp start [ %s ]\n", format_time);

    /* Simulate a simple sleep. This could be logging for many days */
    sleep(10);

    /* Current time after delay print the results */
    time(&end_timestamp);
    time_days = *localtime(&end_timestamp);
    memset(format_time, 0, sizeof format_time);
    strftime(format_time, sizeof format_time, "%c", &time_days);
    printf("timestamp end [ %s ]\n", format_time);

    /* Get the current difference in seconds */
    seconds = difftime(end_timestamp, start_timestamp);
    printf("Seconds elapsed [ %f ]\n", seconds);

    /* Calculate how many days have elapsed */
    cumulative_days = DAY * seconds;

    /* What are we going to do */
    if(cumulative_days <= MAX_DAYS) {
        printf("Delete log and start over\n");
    }
    else {
        printf("Not there yet, keep appending the current log\n");
    }

    return 0;
}

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some bits don't look right...

Code:

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#define HOUR 3600    /* Seconds in one hour */
#define DAY 86400    /* Seconds in one day */

    /* Get the current difference in seconds */
    seconds = difftime(end_timestamp, start_timestamp);
    printf("Seconds elapsed [ %f ]\n", seconds);

    /* Calculate how many days have elapsed */
    cumulative_days = seconds / DAY; //number of days rounded down
    cumulative_days = (seconds + DAY - 1) / DAY; //number of days rounded up

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Thanks for your reply.

I will give that a try and let you know how things turn out.