Arduino timer with nokia display

sketch.ino

#include <Adafruit_GFX.h>
#include <Adafruit_PCD8544.h>

// pin 7 - Serial clock out (SCLK)
// pin 6 - Serial data out (DIN)
// pin 5 - Data/Command select (D/C)
// pin 4 - LCD chip select (CS)
// pin 3 - LCD reset (RST)
Adafruit_PCD8544 display = Adafruit_PCD8544(7, 6, 5, 4, 3);


int rele_pin = 8;
int backlight_pin = 2;

void setup()   {
  Serial.begin(9600);
  pinMode(rele_pin, OUTPUT);
  pinMode(backlight_pin, OUTPUT);
  analogWrite(backlight_pin, 1024);

  display.begin();
  // you can change the contrast around to adapt the display
  // for the best viewing!
  display.setContrast(50);

  display.display(); // show splashscreen
  delay(2000);
  display.clearDisplay();   // clears the screen and buffer

  // text display tests
  display.setTextSize(2);
  display.setTextColor(BLACK);
  display.setCursor(0,0);
}

int time_to_wait = 4 * 60 * 60;
int _3600 = 60 * 60;

void loop() {
  display.clearDisplay();

  if (time_to_wait > 0) {
    digitalWrite(rele_pin, HIGH);
    delay(1000);

    int hours   = time_to_wait / _3600;
    int minutes = (time_to_wait % _3600) / 60;
    int seconds = (time_to_wait % _3600) % 60;

    display.print(hours);
    display.print(":");
    if (minutes < 10) display.print(0);
    display.print(minutes);
    display.print(":");
    if (seconds < 10) display.print(0);
    display.println(seconds);

    time_to_wait--;
  } else {
    analogWrite(backlight_pin, 0);
    digitalWrite(rele_pin, LOW);
    display.println("Done charging!");
  }

  display.display();
}