Arduino Stopwatch

#include <LiquidCrystal.h> //Libraries
LiquidCrystal lcd(2, 3, 4, 5, 6, 7); //Arduino pins to lcd

#define bt_start  A0
#define bt_stop   A1
#define bt_reset  A2

#define G_led 8 
#define R_led 9 

#define buzzer 13

int hh=0, mm=0, ss=0, ms=0; 
bool timerStart = false;

void setup() { // put your setup code here, to run once 

pinMode(bt_start, INPUT_PULLUP);
pinMode(bt_stop,  INPUT_PULLUP);
pinMode(bt_reset, INPUT_PULLUP);

pinMode(R_led,OUTPUT); // declare Red LED as output
pinMode(G_led,OUTPUT); // declare Green LED as output
pinMode(buzzer,OUTPUT); // declare Buzzer as output 

lcd.begin(16, 2); // Configura lcd numero columnas y filas
lcd.clear();
lcd.setCursor (0,0);
lcd.print("  Welcome  To  ");
lcd.setCursor (0,1);
lcd.print("   Stopwatch   ");
delay(2000);
lcd.clear();
 

 noInterrupts();         // disable all interrupts
 TCCR1A = 0;             // set entire TCCR1A register to 0  //set timer1 interrupt at 1kHz  // 1 ms
 TCCR1B = 0;             // same for TCCR1B
 TCNT1  = 0;             // set timer count for 1khz increments
 OCR1A = 1999;           // = (16*10^6) / (1000*8) - 1
 //had to use 16 bit timer1 for this bc 1999>255, but could switch to timers 0 or 2 with larger prescaler
 // turn on CTC mode
 TCCR1B |= (1 << WGM12); // Set CS11 bit for 8 prescaler
 TCCR1B |= (1 << CS11);  // enable timer compare interrupt
 TIMSK1 |= (1 << OCIE1A);
 interrupts();           // enable

}

void loop() {

if(digitalRead (bt_start) == 0){ 
digitalWrite(buzzer, HIGH);   
timerStart = true; // Start stopwatch 
delay(100);
}

if(digitalRead (bt_stop) == 0){ 
digitalWrite(buzzer, HIGH);   
timerStart = false; // Stop stopwatch 
delay(100);
}

if(digitalRead (bt_reset) == 0){ 
digitalWrite(buzzer, HIGH);   
ms=0, ss=0, mm=0, hh=0; // Reset stopwatch 
delay(100);
}

lcd.setCursor (0,0);
lcd.print("   Stopwatch   ");

lcd.setCursor (2,1);
lcd.print((hh/10)%10);
lcd.print(hh%10);
lcd.print(":");
lcd.print((mm/10)%10);
lcd.print(mm%10);
lcd.print(":");
lcd.print((ss/10)%10);
lcd.print(ss%10);
lcd.print(":");
lcd.print((ms/100)%10);
lcd.print((ms/10)%10);
lcd.print(ms%10);

if(timerStart==true){
digitalWrite(G_led, HIGH); // Turn LED on.  
digitalWrite(R_led, LOW);  // Turn LED off.
}else{
digitalWrite(G_led, LOW); // Turn LED off.   
digitalWrite(R_led, HIGH);  // Turn LED on. 
}
  
digitalWrite(buzzer, LOW);
}


ISR(TIMER1_COMPA_vect){   
if(timerStart == true){ms=ms+1;
if(ms>999){ms=0;ss=ss+1;
if(ss>59){ss=0; mm=mm+1;}
if(mm>59){mm=0; hh=hh+1;}
  }  
 }
}