#include <LiquidCrystal_I2C.h>

unsigned long start = millis();
unsigned int startMinutes = 1;
unsigned int startSeconds = 10;
LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  lcd.init();
  lcd.clear();
  lcd.setCursor(0, 0);
}

void loop() {
  timer(startMinutes, startSeconds);
}

void timer(int mins, int secs) {
  static int seconds = secs + 1;
  static int minutes = mins;
  if (millis() - start >= 1000) {
    lcd.clear();
    seconds--;
    char timeStr[6];  // 5 characters + null terminator
    sprintf(timeStr,"%02u:%02u", minutes, seconds);
    lcd.print(timeStr);
    start = millis();
  }
  if ((seconds==0)&&(minutes>=1)){
    minutes = minutes - 1;
    seconds = 60;
  }
  if ((seconds==0)&&(minutes==0)){
    minutes = mins;
    seconds = secs+1;
  }
}