Setting the time on an RTC with an ESP8266 and an NTP server

As we still haven't got rid of this antiquated requirement of adjusting our clocks twice
every year, and as I have a few real-time clocks attached to various mini-projects that
have to be reset every time this happens. I decided it was time to automate this process
a little, using an ESP8266 Node-MCU12-E and I2C.

The wiring is simple:

ESP8266 >>> DS3231
D2 >>> SDA
D1 >>> SCL
G >>> GND
3 >>> VCC

The following code reads the contents of the first 7 DS3231 registers. Then it gets
the current GMT/UTC time from a Network Time Server and performs a local time conversion.
Then it writes the new time to the DS3231 registers and rereads the registers back
to the serial monitor to confirm that the DS3231 has been updated.

/* 
 *  KEYWORDS: DS3231, ESP8266, NodeMCU-12E, NTP
 */

#include <Wire.h>
#include <ESP8266WiFi.h>
#include <NTPClient.h>
#include <WiFiUdp.h>

#define bcd2dec(bcd_in) (bcd_in >> 4) * 10 + (bcd_in & 0x0f)
#define dec2bcd(dec_in) ((dec_in / 10) << 4) + (dec_in % 10)

// Replace with your network credentials
const char* ssid = "your_ssid";
const char* password = "your_password";

// Define NTP Client to get time
WiFiUDP ntpUDP;
NTPClient timeClient(ntpUDP, "pool.ntp.org");

byte mByte[0x07]; // holds the array from the DS3231 register
byte tByte[0x07]; // holds the array from the NTP server

// change the values here to suit your timezone

const long gmtOffset = 3600; // 3600 seconds is +1 hours
const long summerTimeOffset = 3600; // change to 0 in winter

//Week Days
String weekDay[7]={"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};

//Month names
String month[12]={"January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"};

void setup(){
  
  Serial.begin(115200); 
  
  while(!Serial){} // Wait for serial connection

  Wire.begin();
  
  // set up wireless internet connection

  Serial.print("Connecting to ");
  Serial.println(ssid);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("\n");

// Initialize a NTPClient to get the time
  timeClient.begin();
  // Set offset time in seconds to adjust for your timezone, for example:
  // GMT +1 = 3600
  // GMT +8 = 28800
  // GMT -1 = -3600
  // add another 3600 for Summertime if appropriate
  // GMT 0 = 0
  // Example CET = GMT/UTC+1 and CEST = GMT/UTC+2 i.e. 2 x 3600 = 7200
  timeClient.setTimeOffset(gmtOffset + summerTimeOffset); //Central European Summer Time
  
  //get current time from the RTC registers and store in the mByte[] array
  Serial.println(F("Old DS3231 register content........\n"));  
  getRTCdatetime();

  // get the datetime from the NTP server
  
  timeClient.update();

  unsigned long epochTime = timeClient.getEpochTime();  
  tByte[0] = (int)timeClient.getSeconds();  
  tByte[1] = (int)timeClient.getMinutes();  
  tByte[2] = (int)timeClient.getHours();  
  tByte[3] = (int)timeClient.getDay();
  
  // create a struct to hold date, month and year values  
  struct tm *ptm = gmtime ((time_t *)&epochTime);
  tByte[4] = (int)ptm->tm_mday;  
  tByte[5] = (int)ptm->tm_mon+1;  
  tByte[6] = (int)ptm->tm_year-100;

  Serial.print(F("NTP Webtime..........\n\n"));

  Serial.print("Seconds: "); Serial.println(tByte[0]);
  Serial.print("Minutes: "); Serial.println(tByte[1]);
  Serial.print("Hours:   "); Serial.println(tByte[2]);
  Serial.print("DoW:     "); Serial.println(weekDay[tByte[3]]);
  Serial.print("Day:     "); Serial.println(tByte[4]);
  Serial.print("Month:   "); Serial.println(month[tByte[5]-1]);
  Serial.print("Year:    "); Serial.println(tByte[6]);
  Serial.print("\n");

  /* if the time stored in the DS3231 register does not match
   *  the time retrieved from the NTP server, update the DS3231
   *  register to the current time.
   */
  
  if(mByte != tByte){
    Wire.beginTransmission (0x68);
    // Set device to start read reg 0
    Wire.write (0x00);
      for (int idx = 0; idx < 7; idx++){
        Wire.write (dec2bcd(tByte[idx]));
      }   
    Wire.endTransmission ();
    }
    
  Serial.println(F("New DS3231 register content........\n"));  
  getRTCdatetime();
  
}

void loop() {}

void getRTCdatetime(){
  
  Wire.beginTransmission (0x68);
  // Set device to start read reg 0
  Wire.write (0x00); 
  Wire.endTransmission ();
  
  // request 7 bytes from the DS3231 and release the I2C bus
  Wire.requestFrom(0x68, 0x07, true);
  
  int idx = 0;

  // read the first seven bytes from the DS3231 module into the array
  while(Wire.available()) {
    
    byte input = Wire.read(); // read each byte from the register
    mByte[idx] = input;    // store each single byte in the array
    idx++;
  }

  // display the current values to the serial monitor
  
  Serial.print(F("Register[0] Seconds: ")); Serial.println(bcd2dec(mByte[0]));
  Serial.print(F("Register[1] Minutes: ")); Serial.println(bcd2dec(mByte[1]));
  Serial.print(F("Register[2] Hours:   ")); Serial.println(bcd2dec(mByte[2]));
  Serial.print(F("Register[3] DoW:     ")); Serial.println(weekDay[bcd2dec(mByte[3])]);
  Serial.print(F("Register[4] Day:     ")); Serial.println(bcd2dec(mByte[4]));
  Serial.print(F("Register[5] Month:   ")); Serial.println(month[bcd2dec(mByte[5]-1)]);
  Serial.print(F("Register[6] Year:    ")); Serial.println(bcd2dec(mByte[6]));
  Serial.println("\n");


}

I am pretty sure that porting this code to an ESP32, would be reasonably straightforward.