https://www.instructables.com/id/Simplest-ESP8266-Local-Time-Internet-Clock-With-OL/

simplestesp8266clock.ino

// simplestesp8266clock.ino
//
// Libraries needed:
//  Time.h & TimeLib.h:  https://github.com/PaulStoffregen/Time
//  Timezone.h: https://github.com/JChristensen/Timezone
//  SSD1306.h & SSD1306Wire.h:  https://github.com/squix78/esp8266-oled-ssd1306
//  NTPClient.h: https://github.com/arduino-libraries/NTPClient
//  ESP8266WiFi.h & WifiUDP.h: https://github.com/ekstrand/ESP8266wifi
//
// 128x64 OLED pinout:
// GND goes to ground
// Vin goes to 3.3V
// Data to I2C SDA (GPIO 0)
// Clk to I2C SCL (GPIO 2)

// This version has a 24hr display

#include <ESP8266WiFi.h>
#include <WifiUDP.h>
#include <String.h>
#include <Wire.h>
#include <SSD1306.h>
#include <SSD1306Wire.h>
#include <NTPClient.h>
#include <Time.h>
#include <TimeLib.h>
#include <Timezone.h>

// Define NTP properties
#define NTP_OFFSET   60 * 60      // In seconds
#define NTP_INTERVAL 24 * 60 * 60 * 1000    // In miliseconds, once a day
#define NTP_ADDRESS  "ca.pool.ntp.org"  // change this to whatever pool is closest (see ntp.org)

// Set up the NTP UDP client
WiFiUDP ntpUDP;
NTPClient timeClient(ntpUDP, NTP_ADDRESS, NTP_OFFSET, NTP_INTERVAL);

// Create a display object
SSD1306  display(0x3d, 0, 2); //0x3d for the Adafruit 1.3" OLED, 0x3C being the usual address of the OLED
 
const char* ssid = "*************************";   // insert your own ssid 
const char* password = "**********";              // and password
String date;
String t;
const char * days[] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"} ;
const char * months[] = {"Jan", "Feb", "Mar", "Apr", "May", "June", "July", "Aug", "Sep", "Oct", "Nov", "Dec"} ;
const bool show24hr = true;

const char * ampm[] = {"AM", "PM"} ;

// Then convert the UTC UNIX timestamp to local time
TimeChangeRule usEDT = {"EDT", Second, Sun, Mar, 2, -300};  //UTC - 5 hours - change this as needed
TimeChangeRule usEST = {"EST", First, Sun, Nov, 2, -360};   //UTC - 6 hours - change this as needed
Timezone usEastern(usEDT, usEST);
void setup () 
{
  Serial.begin(115200); // most ESP-01's use 115200 but this could vary
  timeClient.begin();   // Start the NTP UDP client

  Wire.pins(0, 2);  // Start the OLED with GPIO 0 and 2 on ESP-01
  Wire.begin(0, 2); // 0=sda, 2=scl
  display.init();
  display.flipScreenVertically();   

  // Connect to wifi
  Serial.println("");
  Serial.print("Connecting to ");
  Serial.print(ssid);
  display.drawString(0, 10, "Connecting to Wifi...");
  display.display();
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) 
  {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.print("Connected to WiFi at ");
  Serial.print(WiFi.localIP());
  Serial.println("");
  display.drawString(0, 24, "Connected.");
  display.display();
}

void loop() 
{
  static time_t local;
  if (WiFi.status() == WL_CONNECTED) //Check WiFi connection status
  {   
    date = "";  // clear the variables
    t = "";
    
    // update the NTP client and get the UNIX UTC timestamp 
    timeClient.update();
    unsigned long epochTime =  ;

    // convert received time stamp to time_t object
    local = usEastern.toLocal(uttimeClient.getEpochTime()c);

    // now format the Time variables into strings with proper names for month, day etc
    date += days[weekday(local)-1];
    date += ", ";
    date += months[month(local)-1];
    date += " ";
    date += day(local);
    date += ", ";
    date += year(local);

    // format the time to 12-hour format with AM/PM and no seconds
    if (show24hr) {
      t += hour(local);
    } else {
      t += hourFormat12(local);
    }
    // blinking colon
    if (second(local) & 1) {
      t += ":";
    } else {
      t += " ";
    }
    if(minute(local) < 10)  // add a zero if minute is under 10
      t += "0";
    t += minute(local);
    if (!show24hr) {
      t += " ";
      t += ampm[isPM(local)];
    }

    // Display the date and time
    Serial.println("");
    Serial.print("Local date: ");
    Serial.print(date);
    Serial.println("");
    Serial.print("Local time: ");
    Serial.print(t);

    // print the date and time on the OLED
    display.clear();
    display.setTextAlignment(TEXT_ALIGN_CENTER);
    display.setFont(ArialMT_Plain_24);
    display.drawStringMaxWidth(64, 10, 128, t);
    display.setFont(ArialMT_Plain_10);
    display.drawStringMaxWidth(64, 38, 128, date);
    display.display();
  }
  else // attempt to connect to wifi again if disconnected
  {
    display.clear();
    display.drawString(0, 10, "Connecting to Wifi...");
    display.display();
    WiFi.begin(ssid, password);
    display.drawString(0, 24, "Connected.");
    display.display();
  }
}