Why does my lcd screen work fine and then display random characters?

Question

It works as expected for a few seconds and then random letters appear on the second row. The wiring and the programming should be correct. I wired it according to an Arduino Starter Kit Book and coded it myself.

Code:

#include <LiquidCrystal.h>
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);
int hours=23;
int minutes=0;
String none="00";
unsigned long timer;
void setup() {
lcd.begin(16, 2);
lcd.setCursor(6, 0);
lcd.print("Time:");
lcd.setCursor(6, 1);
lcd.print("23:00");
}

void loop() {
timer=millis();
if(timer%600==0){
   minutes++;
  if(minutes==60){
      minutes=0;
      hours++;
      }
   if(hours==24){
      hours=0;
   }
  lcd.clear();
  lcd.setCursor(6, 0);
  lcd.print("Time:");
  lcd.setCursor(6, 1);
  if(minutes<10){
    if(hours<10){
      lcd.print(String("0")+hours+":"+"0"+minutes);
    }
    else{
      lcd.print(hours+String(":")+"0"+minutes);
    }
  }
  else{
    if(hours<10){
       lcd.print(String("0")+hours+":"+minutes);
    }
    else{
       lcd.print(hours+":"+minutes);
    }

  }
  delay(200);
  }
}

I was able to narrow down the problem. I was testing what happens when it hits 24 hours so I set hours to 23 in the beginning and made the time 600 ms. Once it hits 10 seconds, the characters appear. If the hour is a 2 digit number, the issue happens. I made the hours 9, and the issue disappeared. What can I do?

Answer 1

The problem was that I was adding ":" to the printed string on the lcd without making it a String object. That only occurred if I had double digit hours and minutes.

Incorrect way:

lcd.print(hours+":"+minutes);

Correct way:

lcd.print(hours+String(":")+minutes);