ESP32: how can`getLocalTime` work without network connection?

Projects Networking, Protocols, and Devices
1

With an ESP32, I’m writing a program to read the status of several sensors, about every 15 minutes with a timestamp. The strategy is to:

  1. connect to the local wifi
  2. get utc timestamp via NTP
  3. read sensor data
  4. disconnect from local wifi
  5. sleeping or doing other stuff for about 15 minutes, then connect to wifi again...

When searching the internet on how to use NTP to get timestamps, the same example code appears on several sites, e.g. ESP32 NTP Client-Server: Get Date and Time (Arduino IDE) | Random Nerd Tutorials

/*
  Rui Santos
  Complete project details at https://RandomNerdTutorials.com/esp32-date-time-ntp-client-server-arduino/
  
  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files.
  
  The above copyright notice and this permission notice shall be included in all
  copies or substantial portions of the Software.
*/

#include <WiFi.h>
#include "time.h"

const char* ssid     = "REPLACE_WITH_YOUR_SSID";
const char* password = "REPLACE_WITH_YOUR_PASSWORD";

const char* ntpServer = "pool.ntp.org";
const long  gmtOffset_sec = 0;
const int   daylightOffset_sec = 3600;

void setup(){
  Serial.begin(115200);

  // Connect to Wi-Fi
  Serial.print("Connecting to ");
  Serial.println(ssid);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.println("WiFi connected.");
  
  // Init and get the time
  configTime(gmtOffset_sec, daylightOffset_sec, ntpServer);
  printLocalTime();

  //disconnect WiFi as it's no longer needed
  WiFi.disconnect(true);
  WiFi.mode(WIFI_OFF);
}

void loop(){
  delay(1000);
  printLocalTime();
}

void printLocalTime(){
  struct tm timeinfo;
  if(!getLocalTime(&timeinfo)){
    Serial.println("Failed to obtain time");
    return;
  }
  Serial.println(&timeinfo, "%A, %B %d %Y %H:%M:%S");
  // cropped other output
}

What I don’t understand in this example is that at the end of the setup part, the wifi connection is terminated. Even more, after WiFi.mode(WIFI_OFF), it seems like the ESP32 has no longer WiFi functionality (WiFi status: 255 == WL_NO_SHEILD), why? None of the websites explaining this example go in to detail about this part of the code :-/

How can a call to getLocalTime (while executing function printLocalTime) in the eternal loop part get the current time if no connection to an NTP-server is present? Is the RTC functionality of the ESP32 used somehow, and if so how is it resynchronised after a while? Apparently getLocalTime is to be called a single time with a wifi connection, and from there on it can do without...

Form within Adruino.cc I tried to get a clue going to the definition of getLocalTime, which lead me to localTime_r, which was a dead end…

Can anyone give me some clues?
Thank you.

2

Hello

Maybe reading this will help you to undestand : Using NTP Server to set time - possible misunderstanding

3

This call returns local time, in contrast to global (UTC) time. Every computer maintains its own clock that is occasionally synced with an NTP server.

4

I was having the same issue - it appears the tutorial/code describes it incorrectly. The getLocalTime() does actually use the internal clock on the device and does not call back to the NTP server

5

I do not depend on NTP (Network Time Protocol), I use it to set my RTC DS3231 then check it against the NTP about once a day. That is checked and updated as needed with UTC. I use that clock for my TimeStamp. So If the network is down my internal clock will be very close.

6

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