difftime

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

int main(void)
{
    time_t now;
    time(&now);

    struct tm beg;
    beg = *localtime(&now);

    // set beg to the beginning of the month
    beg.tm_hour = 0;
    beg.tm_min = 0;
    beg.tm_sec = 0;
    beg.tm_mday = 1;

    double seconds = difftime(now, mktime(&beg));

    printf("%.f seconds have passed since the beginning of the month.\n", seconds);

    return 0;
}