New Year's countdown on an Arduino Uno

// New Year's countdown: a clock for the party that counts down to midnight on
// a four-digit display, then sets off fireworks on a ring of LEDs.
//
//   DS3231    SCL -> A5   SDA -> A4   VCC -> 5 V   GND -> GND
//   TM1637    CLK -> pin 4   DIO -> pin 5   VCC -> 5 V   GND -> GND
//   LED ring  16 WS2812B LEDs, DIN <- 330 ohms <- pin 6, VDD -> 5 V, VSS -> GND
//
// The display shows the time left: hours and minutes until the last hour,
// then minutes and seconds, so the final minute reads 00:59, 00:58 ... 00:00.
// At midnight the ring bursts into fireworks for a minute, and the display
// shows the new year.
//
// The target is worked out from the clock, never typed in: midnight at the
// start of January 1st of the year after the one the clock is in. So the same
// sketch counts down every December 31st, year after year.
//
// Set the clock once: uncomment the rtc.adjust() line, upload, then comment it
// out and upload again. In the simulator the clock starts 15 seconds before
// midnight, so you can watch the moment arrive.
//
// Things to try. Make the fireworks last longer with SHOW_SECONDS. Change the
// colors in PALETTE to your own.

#include <Wire.h>
#include <RTClib.h>
#include <TM1637Display.h>
#include <Adafruit_NeoPixel.h>

const int RING_PIN = 6;
const int LEDS = 16;
const unsigned long SHOW_SECONDS = 60;

RTC_DS3231 rtc;
TM1637Display display(4, 5);   // CLK, DIO
Adafruit_NeoPixel ring(LEDS, RING_PIN, NEO_GRB + NEO_KHZ800);

const uint32_t PALETTE[] = {0xFF2020, 0xFFD040, 0x40A0FF, 0x40FF60, 0xFF40C0, 0xFFFFFF};
const int COLORS = sizeof(PALETTE) / sizeof(PALETTE[0]);

uint32_t lastSecond = 0;
uint32_t showUntil = 0;
unsigned long nextFrame = 0;
uint8_t level[LEDS];
uint32_t color[LEDS];

// Midnight at the start of the next year.
uint32_t nextNewYear(const DateTime &now) {
  return DateTime(now.year() + 1, 1, 1, 0, 0, 0).unixtime();
}

void showLeft(uint32_t seconds) {
  uint32_t h = seconds / 3600;
  uint32_t m = (seconds / 60) % 60;
  uint32_t s = seconds % 60;
  int value = h > 0 ? (int)((h > 99 ? 99 : h) * 100 + m) : (int)(m * 100 + s);
  display.showNumberDecEx(value, 0b01000000, true);   // colon on
}

// One frame of fireworks: now and then a burst of one color lights a few
// LEDs around a random spot, and every LED fades a little each frame.
void fireworks() {
  if (random(0, 6) == 0) {
    int center = random(0, LEDS);
    uint32_t c = PALETTE[random(0, COLORS)];
    for (int d = -2; d <= 2; d++) {
      int i = (center + d + LEDS) % LEDS;
      level[i] = 255 - abs(d) * 60;
      color[i] = c;
    }
  }
  for (int i = 0; i < LEDS; i++) {
    uint8_t r = (uint8_t)(color[i] >> 16), g = (uint8_t)(color[i] >> 8), b = (uint8_t)color[i];
    ring.setPixelColor(i, r * level[i] / 255, g * level[i] / 255, b * level[i] / 255);
    level[i] = level[i] > 12 ? level[i] - 12 : 0;
  }
  ring.show();
}

void setup() {
  Serial.begin(9600);
  display.setBrightness(5);
  ring.begin();
  ring.setBrightness(180);
  ring.show();
  if (!rtc.begin()) {
    Serial.println("No DS3231 answered: check SDA and SCL");
    while (true) {
    }
  }
  // rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));   // set once, see above
  randomSeed(analogRead(A0));
  Serial.println("New Year's countdown");
}

void loop() {
  if (showUntil > 0 && millis() >= nextFrame) {
    nextFrame = millis() + 30;
    fireworks();
  }

  DateTime now = rtc.now();
  uint32_t t = now.unixtime();
  if (t == lastSecond) return;
  lastSecond = t;

  // The first minute of January 1st: the show, and the new year on the display.
  if (now.month() == 1 && now.day() == 1 && now.hour() == 0 && now.minute() == 0) {
    if (showUntil == 0) {
      showUntil = t + SHOW_SECONDS;
      Serial.print("Happy New Year! Welcome to ");
      Serial.println(now.year());
    }
    display.showNumberDec(now.year(), false);
    return;
  }
  if (showUntil > 0 && t >= showUntil) {
    showUntil = 0;
    ring.clear();
    ring.show();
    Serial.println("Show over, counting to next year");
  }

  uint32_t left = nextNewYear(now) - t;
  showLeft(left);
  if (left <= 10) {
    Serial.println(left);
  } else if (now.second() == 0) {
    Serial.print(left / 60);
    Serial.println(" minutes to go");
  }
}