./ps/display.c
-------------------------------------
int main(int argc, char *argv[]){
...
reset_global();
...
}
-------------------------------------

./ps/global.c
-------------------------------------
void reset_global(void){
...
 /* get boot time from /proc/stat */
  fd = open("/proc/stat", O_RDONLY, 0);
  if (fd != -1) {
    buf[BUFFSIZE-1] = 0;
    read(fd, buf, BUFFSIZE-1);
    b = strstr(buf, "btime ");
    if (b) {
      sscanf(b, "btime %lu", &time_of_boot);
      seconds_since_boot = time(0) - time_of_boot;
    }
    close(fd);
  }
...
}
-------------------------------------

./ps/output.c
-------------------------------------
static int pr_stime(char *restrict const outbuf, const proc_t *restrict const pp){
...
t = time_of_boot + pp->start_time / Hertz;
...
}
-------------------------------------