#include <Wire.h>
#include <RTClib.h>
#include "Adafruit_GFX.h"
#include "Adafruit_SSD1306.h"
#define SCREEN_WIDTH  64
#define SCREEN_HEIGHT 64
Adafruit_SSD1306 oled(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, -1);
RTC_DS1307 rtc;
String time;

void setup() {
  Serial.begin(9600);
  if (!oled.begin(SSD1306_SWITCHCAPVCC, 0x3C)) 
  {
    Serial.println(F("SSD1306 allocation failed"));
    while (true);
  }
  delay(2000);
  oled.clearDisplay();
  oled.setTextSize(2);
  oled.setTextColor(WHITE);
  oled.setCursor(0, 10);
  if (! rtc.begin()) {
    Serial.println("errore rtc");
    Serial.flush();;
    while (true);
  }
  rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
  time.reserve(10);;
}

void loop() 
{
  DateTime now = rtc.now();
  time = "";
  time += now.hour();
  time += ':';
  time += now.minute();
  time += ':';
  time += now.second();
  oledDisplayCenter(time);
}

void oledDisplayCenter(String text) 
{
  int16_t x1;
  int16_t y1;
  uint16_t width;
  uint16_t height;
  oled.getTextBounds(text, 0, 0, &x1, &y1, &width, &height);
  oled.clearDisplay(); // clear display
  oled.setCursor((SCREEN_WIDTH - width)/1, (SCREEN_HEIGHT - height)/2);
  oled.println(text); // text to display
  oled.display();
}