Comparing dates in C with (Using time.h library)

Question

hi there i can compare people birthday in format YYYY-MM-DD with string (strcmp) functions. but i need compare todays date with person's birthday to display if his/her birthday is in 7 days or not_?. i searched "time.h" library but couldn't managed it. i appreciated if you can help.

Answer 1

I would use difftime on the time_t values and compare against the number of seconds in a week...

Answer 2

The following sample program converts a given string on the argument line to the number of days. Example output:

% ./nd 2011-06-18 1971-02-10 invalid 2010invalid
38 days
-14699 days
2147483647 days
2147483647 days

Edit: Decided that -1 is not a nice failure indicator so using INX_MAX instead.

#include <sys/param.h>
#include <time.h>
#include <string.h>
#include <stdio.h>

#define ONE_DAY (24 * 3600)

int main(int argc, char *argv[])
{
        int i;

        if( argc < 2 )
                return (64); // EX_USAGE

        for( i=1; i < argc; i++ )
        {
                time_t res;

                res = num_days(argv[i]);
                printf("%d days\n", res);
        }

        return(0);
}

int num_days(const char *date)
{
        time_t now = time(NULL);
        struct tm tmp;
        double res;

        memset(&tmp, 0, sizeof(struct tm));
        if( strptime(date, "%F", &tmp) == NULL )
                return INT_MAX;

        res = difftime(mktime(&tmp), now);
        return (int)(res / ONE_DAY);
}