void setup(){
  DDRD = 0b11111111;
  DDRB = 0b111111;
  DDRC = 0b11111;
}

void loop(){
  static byte sec = 0, min = 0, hour = 0;
  static byte sE, mE, hE;
  static byte sZ, mZ, hZ;

  delay(250);
  sec++;
  if(sec >= 60){
    sec = 0;
    min++;
    if(min >= 60){
      min = 0;
      hour++;
      if(hour >= 24) hour = 0;
    }
  }
  PORTD = sec;
  PORTB = min;
  PORTC = hour;
}