Nixie Clock Countdown code based on GreatScott!'s code for nixie clock

NixieClockCountdown.ino

#include <Wire.h>
#define A1 3
#define B1 4
#define C1 5
#define D1 6
#define A2 7
#define B2 8
#define C2 9
#define D2 10
#define A3 11
#define B3 12
#define C3 13
#define D3 14
#define A4 15
#define B4 16
#define C4 2
#define D4 1
#define pot A3
char A[4] = {A1, A2, A3, A4};
char B[4] = {B1, B2, B3, B4};
char C[4] = {C1, C2, C3, C4};
char D[4] = {D1, D2, D3, D4};
int zero;
int one;
int two;
int three;

int timeTo = 3600;

void setup() {
  pinMode(A1, OUTPUT);
  pinMode(B1, OUTPUT);
  pinMode(C1, OUTPUT);
  pinMode(D1, OUTPUT);
  pinMode(A2, OUTPUT);
  pinMode(B2, OUTPUT);
  pinMode(C2, OUTPUT);
  pinMode(D2, OUTPUT);
  pinMode(A3, OUTPUT);
  pinMode(B3, OUTPUT);
  pinMode(C3, OUTPUT);
  pinMode(D3, OUTPUT);
  pinMode(A4, OUTPUT);
  pinMode(B4, OUTPUT);
  pinMode(C4, OUTPUT);
  pinMode(D4, OUTPUT);
  pinMode(pot, INPUT);
  for (char i = 0; i < 4; i++) {
    digitalWrite(A[i], HIGH);
    digitalWrite(B[i], HIGH);
    digitalWrite(C[i], HIGH);
    digitalWrite(D[i], HIGH);
  }
}

void loop() {
  int nDigits = floor(log10(abs(timeTo))) + 1;
  
  switch (nDigits){
    case 1:
      zero = 0;
      one = 0;
      two =  0;
      three = timeTo % 10;
      break;
    case 2:
      zero = 0;
      one = 0;
      two =  (timeTo / 10) % 10;
      three = timeTo % 10;
      break;
    case 3:
      zero = 0;
      one = timeTo % 1000 / 100;
      two =  (timeTo / 10) % 10;
      three = timeTo % 10;
      break;
    case 4:
      zero = (timeTo / 1000) % 1000;
      one = timeTo % 1000 / 100;
      two =  (timeTo / 10) % 10;
      three = timeTo % 10;
      break;
  }

//  Serial.println(zero);
//  Serial.println(one);
//  Serial.println(two);
//  Serial.println(three);
  writenumber(0, zero);
  writenumber(1, one);
  writenumber(2, two);
  writenumber(3, three);

  timeTo--;
  
  delay(1000);

}

void writenumber(int a, int b) {
  switch (b) {
    case 0:
      digitalWrite(A[a], LOW);
      digitalWrite(B[a], LOW);
      digitalWrite(C[a], LOW);
      digitalWrite(D[a], LOW);
      break;
    case 9:
      digitalWrite(A[a], HIGH);
      digitalWrite(B[a], LOW);
      digitalWrite(C[a], LOW);
      digitalWrite(D[a], LOW);
      break;
    case 8:
      digitalWrite(A[a], LOW);
      digitalWrite(B[a], HIGH);
      digitalWrite(C[a], LOW);
      digitalWrite(D[a], LOW);
      break;
    case 7:
      digitalWrite(A[a], HIGH);
      digitalWrite(B[a], HIGH);
      digitalWrite(C[a], LOW);
      digitalWrite(D[a], LOW);
      break;
    case 6:
      digitalWrite(A[a], LOW);
      digitalWrite(B[a], LOW);
      digitalWrite(C[a], HIGH);
      digitalWrite(D[a], LOW);
      break;
    case 5:
      digitalWrite(A[a], HIGH);
      digitalWrite(B[a], LOW);
      digitalWrite(C[a], HIGH);
      digitalWrite(D[a], LOW);
      break;
    case 4:
      digitalWrite(A[a], LOW);
      digitalWrite(B[a], HIGH);
      digitalWrite(C[a], HIGH);
      digitalWrite(D[a], LOW);
      break;
    case 3:
      digitalWrite(A[a], HIGH);
      digitalWrite(B[a], HIGH);
      digitalWrite(C[a], HIGH);
      digitalWrite(D[a], LOW);
      break;
    case 2:
      digitalWrite(A[a], LOW);
      digitalWrite(B[a], LOW);
      digitalWrite(C[a], LOW);
      digitalWrite(D[a], HIGH);
      break;
    case 1:
      digitalWrite(A[a], HIGH);
      digitalWrite(B[a], LOW);
      digitalWrite(C[a], LOW);
      digitalWrite(D[a], HIGH);
      break;
  }
}

void off(int a) {
  digitalWrite(A[a], HIGH);
  digitalWrite(B[a], HIGH);
  digitalWrite(C[a], HIGH);
  digitalWrite(D[a], HIGH);
}