閏秒にあわせて作成しました。100ミリ秒ごとにミリ秒単位で時刻を表示し続けます。

gistfile1.sh

#!/bin/sh

while sleep 1
do
  ntpq -c rv; echo
done

Makefile

all: ptime

ptime.o: ptime.c

ptime: ptime.o
	clang++ -o $@ $^

clean:
	rm -f ptime ptime.o

ptime.c

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

void ptime() {
  struct timeval tv;
  struct tm* ptm;
  char time_string[40];
  long milliseconds;

  /* Obtain the time of day, and convert it to a tm struct.  */
  gettimeofday (&tv, NULL);
  ptm = localtime (&tv.tv_sec);
  /* Format the date and time, down to a single second.  */
  strftime (time_string, sizeof (time_string), "%Y-%m-%d %H:%M:%S", ptm);
  /* Compute milliseconds from microseconds.  */
  milliseconds = tv.tv_usec / 1000;
  /* Print the formatted time, in seconds, followed by a decimal point
   *      and the milliseconds.  */
  printf ("%s.%03ld\n", time_string, milliseconds);
}

int main() {
  while (1) {
    ptime();
    usleep(100000);
  }
}