//en.cppreference.com/w/c/chrono/difftime.html
#include <stdio.h>
#include <time.h>
int main(void)
{
time_t now = time(0);
struct tm 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;
}