We can sync the time of a ESP-8266 with a NTP server and then keep time count on the device. It will not need any other additional devices like a DS1307 clock. And once synced with the NTP server it can keep the time even if there is no internet  – very handy.

The time formatting and display functions are standard C++ functions.

#include <ESP8266WiFi.h>

#include "TinyDHT.h"

const char *ssid     = "xxxxx";
const char *password = "xxxxxx";


#define DHTPIN11 14          // What digital pin we're connected to
#define DHTTYPE11 DHT11     // DHT 11

DHT dht11(DHTPIN11, DHTTYPE11);

struct tm* tm;

float temperature, relHumidity;
String currentTime;

#define countof(a) (sizeof(a) / sizeof(a[0]))

#define LED 13

void setup() {

  time_t now = 0;

  // Initialize serial and wait for port to open:
  Serial.begin(9600);
  // This delay gives the chance to wait for a Serial Monitor without blocking if none is found
  delay(1500); 

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

  Serial.println("WiFi connected");
  
//sync time with the NTP server. First param 19800 seconds = +5:30 for IST, Second Param 0 = DST 
 configTime(19800, 0, "pool.ntp.org");

//waiting for time to get synced before proceeding. This is not strictly required. Please see after code 
  do
  {
    now = time(nullptr);
    delay(500);
    Serial.print("*");
  }while(now < time(nullptr));
  
  pinMode(LED, OUTPUT);
  
  pinMode(DHTPIN11, INPUT);
  dht11.begin();

  onLedIndicatorChange();
}


void loop() {
  // Your code here 
  time_t now = time(nullptr);
  
  char datestring[30];
  
  strftime(datestring, 
            countof(datestring),
            "%d/%m/%Y %I:%M:%S %p",
            localtime(&now)
            );
   
  currentTime = datestring; 
  Serial.println(currentTime);
  temperature = dht11.readTemperature();
  delay(200);
  relHumidity = dht11.readHumidity();
  delay(200);

}

void onLedIndicatorChange() {
  // Do something
  int ledIndicator = 1; 
  digitalWrite(LED, ledIndicator);
}

configtime listens for connection in the back and will sync as soon as a connection is available. So if a time is not critical for the successive codes or functions then the loop that waits for the sync can be skipped.

configTime(19800, 0, "pool.ntp.org");
do //loop and wait for a sync
{
  now = time(nullptr);
  delay(500);
  Serial.print("*");
}while(now < time(nullptr));