Arduino Reflex Tester

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

#define ledPin 8
#define btnPin 4

double number;
long sTime;

LiquidCrystal_I2C lcd(0x3F, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);

void setup() {
  Serial.begin(9600);
  lcd.begin(16,2);
  lcd.setCursor(0,0);
  pinMode(btnPin, INPUT);
  pinMode(ledPin, OUTPUT);
}

void loop() {
    number = random(500, 5000);
    lcd.print("Wait for the LED");
    delay(number);

    lcd.clear();
    lcd.print("     PRESS!");
    digitalWrite(ledPin, HIGH);
    sTime = millis();

    while (digitalRead(btnPin) == LOW);

    lcd.clear();
    lcd.print(" Reaction time: ");
    lcd.setCursor(0,1);
    lcd.print("      ");
    lcd.print((millis() - sTime) / 1000.0);
    lcd.print(" s");

    delay(2000);

    lcd.clear();
    lcd.setCursor(0,0);
    lcd.print("  Another test");
    lcd.setCursor(0, 1);
    lcd.print("  in 5 seconds");
    delay(5000);
    lcd.clear();
    digitalWrite(8, LOW);
}