Christmas countdown on an Arduino Uno

// Christmas countdown: a display for the front window that counts down the
// days, hours, minutes and seconds to Christmas morning, then flashes the
// date all through Christmas Day.
//
//   DS3231    SCL -> SCL   SDA -> SDA   VCC -> 5 V   GND -> GND
//   MAX7219   8-digit seven-segment module: DIN -> pin 12, CLK -> pin 11,
//             CS -> pin 10, VCC -> 5 V, GND -> GND
//
// The display reads DD.HH.MM.SS: days on the left two digits, then hours,
// minutes and seconds, each pair split by a decimal point.
//
// Why a real-time clock: the Uno's own millis() forgets everything at every
// power cut and drifts over weeks. The DS3231 has a coin cell and a
// temperature-compensated crystal, so it keeps the date through a blown fuse
// on the porch and stays close to the right time all season. Set it once:
// uncomment the rtc.adjust() line, upload, then comment it out and upload
// again, or every reset will put the clock back to the moment you built it.
//
// The countdown is plain arithmetic on DateTime: Christmas minus now is a
// TimeSpan, and TimeSpan already knows how to split itself into days, hours,
// minutes and seconds.
//
// On the bench: turn INTENSITY down for a window at night; full brightness
// is a lot of light through glass.
//
// Things to try. Count to New Year's Eve instead by changing the month and
// day in christmasAfter(). Show only the days when there are more than
// thirty of them, so it changes once a day.

#include <Wire.h>
#include <RTClib.h>
#include <LedControl.h>

const int INTENSITY = 6;   // 0 to 15

RTC_DS3231 rtc;
LedControl display(12, 11, 10, 1);   // DIN, CLK, CS, one module

uint32_t lastShown = 0;

// The next Christmas morning that has not yet ended.
DateTime christmasAfter(const DateTime &now) {
  DateTime target(now.year(), 12, 25, 0, 0, 0);
  if (now.unixtime() >= target.unixtime() + 86400UL) {
    target = DateTime(now.year() + 1, 12, 25, 0, 0, 0);
  }
  return target;
}

// Two digits at positions `left` and `left - 1`, with an optional point.
void pair(int left, int value, bool point) {
  display.setDigit(0, left, value / 10, false);
  display.setDigit(0, left - 1, value % 10, point);
}

void showCountdown(const TimeSpan &left) {
  int days = left.days();
  if (days > 99) days = 99;
  pair(7, days, true);
  pair(5, left.hours(), true);
  pair(3, left.minutes(), true);
  pair(1, left.seconds(), false);
}

void showChristmas(const DateTime &now) {
  // 25.12.2026, on for a second and off for a second.
  if (now.second() % 2 == 0) {
    pair(7, 25, true);
    pair(5, 12, true);
    pair(3, now.year() / 100, false);
    pair(1, now.year() % 100, false);
  } else {
    display.clearDisplay(0);
  }
}

void setup() {
  Serial.begin(9600);
  display.shutdown(0, false);
  display.setIntensity(0, INTENSITY);
  display.clearDisplay(0);
  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
  if (rtc.lostPower()) Serial.println("The clock has lost power: set it once with rtc.adjust()");
  Serial.println("Christmas countdown");
}

void loop() {
  DateTime now = rtc.now();
  if (now.unixtime() == lastShown) return;   // once a second is enough
  lastShown = now.unixtime();

  DateTime christmas = christmasAfter(now);
  if (now.unixtime() >= christmas.unixtime()) {
    showChristmas(now);
    if (now.second() == 0) Serial.println("Merry Christmas!");
    return;
  }
  TimeSpan left = christmas - now;
  showCountdown(left);
  Serial.print(left.days());
  Serial.print(" days ");
  Serial.print(left.hours());
  Serial.print(" h ");
  Serial.print(left.minutes());
  Serial.print(" min ");
  Serial.print(left.seconds());
  Serial.println(" s to go");
}