Arduino ISR Clock

ISRClock.ino

#include <TimerOne.h>
 
int hour = 0;
int minute = 0;
int second = 01;
int latchPin = A2;
int clockPin = A1;
int dataPin = A0;

byte numbers[] = {
B11111101,
B01100001,
B11011011,
B11110011,
B01100111,
B10110111,
B10111111,
B11100001,
B11111111,
B11110111
};

void setup() 
{
  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, OUTPUT);
  Timer1.initialize(1000000); 
  Timer1.attachInterrupt( timerIsr );
  Serial.begin(57600);
}

void loop()
{
    if(Serial.available() >= 4)
    {
      int action = Serial.read();
      int newhour = Serial.read();
      int newminute = Serial.read();
      int newsecond = Serial.read();
      if (action == 1)
      {
        hour = newhour;
        minute = newminute;
        second = newsecond;
        Serial.write(15);
      }
    }
    digitalWrite(latchPin, LOW);
    char chSecond[2];
    sprintf(chSecond, "%02d", second);
    char chMinute[2];
    sprintf(chMinute, "%02d", minute);
    char chHour[2];
    sprintf(chHour, "%02d", hour);
    shiftOut(dataPin, clockPin, MSBFIRST, ~numbers[(int)chSecond[1] - 48]);  //Sekunde 2
    shiftOut(dataPin, clockPin, MSBFIRST, ~numbers[(int)chSecond[0] - 48]);  //Sekunde 1
    shiftOut(dataPin, clockPin, MSBFIRST, ~numbers[(int)chMinute[1] - 48]);  //Minute 2
    shiftOut(dataPin, clockPin, MSBFIRST, ~numbers[(int)chMinute[0] - 48]);  //Minute 1
    shiftOut(dataPin, clockPin, MSBFIRST, ~numbers[(int)chHour[1] - 48]);  //Stunde 2
    shiftOut(dataPin, clockPin, MSBFIRST, ~numbers[(int)chHour[0] - 48]);  //Stunde 1
    digitalWrite(latchPin, HIGH);
    delay(10);
}

void timerIsr()
{
    second++;
   if(second == 60)
   {
     second = 0;
     minute++;
   }
   if(minute == 60)
   {
     minute = 0;
     hour++;
   }
   if(hour == 24)
   {
     hour = 0;
   }
}