wdrift.c

#include <stdlib.h>
#include <stdio.h>
#include <sys/time.h>

int main(void)
{
    struct timeval now;
    double seconds;

    gettimeofday(&now, NULL);

    /*
      To keep the implementation simple, we assume the absolute value of the
      drift is less than 30s and work with the distance to the current minute.
      This is enough for me, but can be improved by asking the user which was
      the time in their watch.
    */
    seconds = (now.tv_sec % 60) + now.tv_usec/1e6;

    if (seconds < 30) {
        /* Past real 00s means we are late, the watch is slow. */
        printf("-%0.2f\n", seconds);
    } else {
        /* Otherwise, the watch is on time, or going too fast. */
        printf("+%0.2f\n", 60 - seconds);
    }

    return EXIT_SUCCESS;
}