Clock not keeping time

Projects General Guidance
1

Hi, I'm new to programming bought an Arduino to create some real-world projects.
I'm trying to code a clock with just a UNO R3 started kit. I'm using the millie() function to keep time. However, after 10 minutes, there is a delay of about a second.

#include <LiquidCrystal.h>

LiquidCrystal lcd(7, 8, 9, 10, 11, 12); 
int seconds = 0;
int minutes = 0; 
int hours = 0; 
unsigned long tSeconds = 1000; 
unsigned long recTime = 0; 

void setup() {
  int hours = 0; 
  lcd.begin(16, 2);
  lcd.print("Clock");
  
  lcd.setCursor(8,1);
  lcd.print("  :  :  ");
}

void loop() {
  
 
  lcd.setCursor(0, 1);
  lcd.print(millis()/1000);
  lcd.setCursor(14, 1);
   if (seconds < 10){
    lcd.print("0");
  }
  lcd.print(seconds);
  
lcd.setCursor(11, 1);
  if (minutes < 10){
    lcd.print("0");
  }
  lcd.print(minutes);
  lcd.setCursor(8, 1);
  if (hours < 10){
    lcd.print("0");
  }
  lcd.print(hours);
  
  if (millis() - recTime >= tSeconds){
    seconds += 1; 
    recTime = millis();
  }
  if (seconds >= 60){
    minutes += 1; 
    seconds = 0; 
  }
  if (minutes >= 60){
    hours += 1; 
    minutes = 0; 
  }
  if (hours >= 24){
    hours = 0;
  }
 }
2

Your code is unnecessarily complex. Just grab the current value of millis and calculate how many hours, minutes and seconds it is.

This is risky:

  if (millis() - recTime >= tSeconds){
    seconds += 1; 
    recTime = millis();
  }

millis may tick over while you're incrementing seconds and now you've got time unaccounted for.

3

Read all about millis().

4

In any case a clock built around millis() will not be very accurate , ideally use something like a DS3231