Pomodoro timer on an active buzzer (pin 9), on an Arduino Uno

// A Pomodoro timer: 25 minutes of work, 5 of rest, on a buzzer on pin 9.
//
//   pin 9 -> the active buzzer's + leg; the other leg -> GND
//
// delay() would freeze the board for 25 minutes, so the timer keeps time with
// millis() instead: the milliseconds since the board started. It remembers
// when the current period began and switches when enough time has passed,
// with the loop free to do other things (here, print the minutes left).

const int BUZZER = 9;
const unsigned long WORK_MS = 25UL * 60UL * 1000UL;
const unsigned long BREAK_MS = 5UL * 60UL * 1000UL;

bool working = true;
unsigned long started = 0;
long lastMinute = -1;

void beep(int times, int ms) {
  for (int i = 0; i < times; i++) {
    digitalWrite(BUZZER, HIGH);
    delay(ms);
    digitalWrite(BUZZER, LOW);
    delay(200);
  }
}

void setup() {
  pinMode(BUZZER, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: Pomodoro timer, 25 minutes of work, then 5 of rest");
  beep(1, 600);
  started = millis();
}

void loop() {
  unsigned long period = working ? WORK_MS : BREAK_MS;
  unsigned long elapsed = millis() - started;
  if (elapsed >= period) {
    working = !working;
    started = millis();
    lastMinute = -1;
    if (working) beep(1, 600);
    else beep(2, 200);
    Serial.println(working ? "back to work" : "break time");
    return;
  }
  long minutesLeft = (period - elapsed + 59999UL) / 60000UL;
  if (minutesLeft != lastMinute) {
    lastMinute = minutesLeft;
    Serial.print(working ? "work: " : "break: ");
    Serial.print(minutesLeft);
    Serial.println(" min left");
  }
}