I2C LCD clock and thermometer on an Arduino Uno

// A clock and thermometer on an I2C LCD.
//
//   LCD GND -> GND   VCC -> 5V   SDA -> A4   SCL -> A5
//   10k from 5V -> A0; the thermistor from A0 -> GND
//
// MsTimer2 calls tick() every 1000 ms from a timer interrupt, so the seconds
// keep counting however long the rest of the loop takes. Type three numbers,
// like 12,30,0, into the serial monitor to set the time.

#include <Wire.h>
#include <LiquidCrystal_I2C.h>
#include <MsTimer2.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);
volatile int seconds = 0;
int hour = 0, minute = 0;

// The thermistor is the bottom of a divider under a 10k, so its resistance
// is 10k * v / (5 - v). The beta equation turns that into kelvin:
// 1/T = 1/T0 + ln(R/R0)/B, with T0 = 25 C, R0 = 10k and B = 3950.
float celsius() {
  float v = analogRead(A0) * 5.0 / 1024.0;
  float rt = 10.0 * v / (5.0 - v);
  float kelvin = 1.0 / (log(rt / 10.0) / 3950.0 + 1.0 / (273.15 + 25.0));
  return kelvin - 273.15;
}

void tick() {
  seconds++;
}

void twoDigits(int n) {
  lcd.print(n / 10);
  lcd.print(n % 10);
}

void setup() {
  lcd.init();
  lcd.backlight();
  lcd.print("starting...");
  delay(500);
  lcd.clear();
  MsTimer2::set(1000, tick);
  MsTimer2::start();
  Serial.begin(9600);
  Serial.println("Type hour,minute,second to set the time.");
}

void loop() {
  if (Serial.available() > 0) {
    int h = Serial.parseInt();
    int m = Serial.parseInt();
    int s = Serial.parseInt();
    hour = constrain(h, 0, 23);
    minute = constrain(m, 0, 59);
    noInterrupts();
    seconds = constrain(s, 0, 59);
    interrupts();
    Serial.println("time set");
  }
  noInterrupts();
  if (seconds >= 60) {
    seconds -= 60;
    if (++minute >= 60) {
      minute = 0;
      if (++hour >= 24) hour = 0;
    }
  }
  int sec = seconds;
  interrupts();
  lcd.setCursor(0, 0);
  lcd.print("TEMP: ");
  lcd.print(celsius());
  lcd.print("C ");
  lcd.setCursor(0, 1);
  lcd.print("TIME: ");
  twoDigits(hour);
  lcd.print(':');
  twoDigits(minute);
  lcd.print(':');
  twoDigits(sec);
  delay(200);
}