timer.c

#include <stdlib.h>
#include <unistd.h>
#include <time.h>
#include <string.h>
#include <signal.h>
#include <curses.h>

void quit()
{
	endwin();
	exit(EXIT_SUCCESS);
}

void sighandler(int sig)
{
	quit();
}

int digits[13][7] =
	{
		{ 0, 0, 0, 0, 0,14,14},
		{ 0, 0, 0, 0, 0, 0, 0},
		{14,17,17,17,17,17,14},
		{ 4, 6, 5, 4, 4, 4,31},
		{14,17, 8, 4, 2, 1,31},
		{14,17,16,14,16,17,14},
		{17,17,17,31,16,16,16},
		{31, 1, 1,15,16,17,14},
		{14,17, 1,15,17,17,14},
		{31,16,16, 8, 4, 4, 4},
		{14,17,17,14,17,17,14},
		{14,17,17,30,16,17,14},
		{ 0,14,14, 0,14,14, 0}
	};

void drawdigit(int y, int x, char c)
{
	int *digit = digits[c - '.'];
	int i, j;
	for(i = 0; i < 7; i++)
	{
		int v = digit[i];
		for(j = 0; j < 5; j++)
			if(v & (1 << j))
			{
				attron(COLOR_PAIR(1));
				mvaddch(y + i, x + j, ' ');
				attroff(COLOR_PAIR(1));
			}
			else
				mvaddch(y + i, x + j, ' ');
	}
}

struct timespec now;
struct tm last;
int newsecond;

void updatetime()
{
	newsecond = 0;
	time_t old = now.tv_sec;
	clock_gettime(CLOCK_REALTIME, &now);
	if(now.tv_sec != old)
	{
		memcpy(&last, gmtime(&now.tv_sec), sizeof(struct tm));
		newsecond = 1;
	}
}

void drawtime()
{
	int d = 30;
	int y = 18;
	if(newsecond)
	{
		drawdigit(y, d + 1, last.tm_hour / 10 + '0');
		drawdigit(y, d + 7, last.tm_hour % 10 + '0');
		drawdigit(y, d + 13, ':');
		drawdigit(y, d + 19, last.tm_min / 10 + '0');
		drawdigit(y, d + 25, last.tm_min % 10 + '0');
		drawdigit(y, d + 31, ':');
		drawdigit(y, d + 37, last.tm_sec / 10 + '0');
		drawdigit(y, d + 43, last.tm_sec % 10 + '0');
		drawdigit(y, d + 49, '.');
	}
	drawdigit(y, d + 55, now.tv_nsec / 100000000 + '0');
	drawdigit(y, d + 61, now.tv_nsec / 10000000 % 10 + '0');
	drawdigit(y, d + 67, now.tv_nsec / 1000000 % 10 + '0');
}

time_t NEW_YEAR = 1483228800;
int zones[] = {840, 825, 780, 765, 720, 690, 660, 630, 600, 570, 540, 510, 480, 420, 390, 360, 345, 330, 300, 270, 240, 210, 180, 120, 60, 0, -60, -120, -180, -210, -240, -300, -360, -420, -480, -540, -570, -600, -660, -720};
int nzones = sizeof(zones) / sizeof(*zones);

int drawzone(int y, int x, int offt)
{
	if(newsecond)
	{
		move(y, x);
		if(now.tv_sec + offt * 60 >= NEW_YEAR)
		{
			attron(COLOR_PAIR(2));
			printw("UTC%c%02d%02d | ------------", offt >= 0 ? '+' : '-', abs(offt) / 60, abs(offt) % 60);
			attroff(COLOR_PAIR(2));
		}
		else
		{
			attron(COLOR_PAIR(3));
			printw("UTC%c%02d%02d | ", offt >= 0 ? '+' : '-', abs(offt) / 60, abs(offt) % 60);
			time_t left_sec = NEW_YEAR - now.tv_sec - offt * 60 - 1;
			if(left_sec >= 3600)
				printw("%2ld:%02ld:%02ld.%03ld", left_sec / 3600, left_sec / 60 % 60, left_sec % 60, (1000000000 - now.tv_nsec) / 1000000 % 1000);
			else if(left_sec >= 60)
				printw("   %2ld:%02ld.%03ld", left_sec / 60 % 60, left_sec % 60, (1000000000 - now.tv_nsec) / 1000000 % 1000);
			else
				printw("      %2ld.%03ld", left_sec % 60, (1000000000 - now.tv_nsec) / 1000000 % 1000);
			attroff(COLOR_PAIR(3));
		}
	}
	else
	{
		if(now.tv_sec + offt * 60 < NEW_YEAR)
		{
			attron(COLOR_PAIR(3));
			move(y, x + 20);
			printw("%03ld", (1000000000 - now.tv_nsec) / 1000000 % 1000);
			attroff(COLOR_PAIR(3));
		}
	}
}

int main()
{
	initscr();
	noecho();
	start_color();
	init_pair(1, COLOR_BLACK, COLOR_WHITE);
	init_pair(2, COLOR_RED, COLOR_BLACK);
	init_pair(3, COLOR_GREEN, COLOR_BLACK);
	signal(SIGINT, sighandler);

	while(1)
	{
		updatetime();
		drawtime();
		int i;
		for(i = 0; i < nzones; i++)
			drawzone(i + 1, 1, zones[i]);
		refresh();
		usleep(500);
	}
	
	pause();
	quit();
}

on line 150, I find the below to be sufficient, and less taxing on CPU:
usleep(50000);