ESP32_NTP_Callback.ino

#include <WiFi.h>

// https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/system/system_time.html#

// https://github.com/espressif/esp-idf/blob/master/examples/protocols/sntp/main/sntp_example_main.c

const char* WIFI_NETWORK =         "xxx";
const char* WIFI_PASSWORD =        "xxx";

const char* NTP_POOL =             "nl.pool.ntp.org";
const char* TIMEZONE =             "CET-1CEST,M3.5.0/2,M10.5.0/3"; /* Central European Time - see http://www.remotemonitoringsystems.ca/time-zone-abbreviations.php */

void print_time(struct timeval *tv)
{
  struct tm localTime;
  localtime_r(&tv->tv_sec, &localTime);

  // https://www.cplusplus.com/reference/ctime/strftime/
  char strftime_buf[64];
  strftime(strftime_buf, sizeof(strftime_buf), "%T", &localTime);

  ESP_LOGI(TAG, "ntp sync at %s:%i", strftime_buf, tv->tv_usec);
}

void setup() {
  WiFi.begin(WIFI_NETWORK, WIFI_PASSWORD);
  WiFi.setSleep(false);

  while (!WiFi.isConnected())
    delay(10);

  ESP_LOGI(TAG, "connected to '%s' as %s - syncing NTP", WIFI_NETWORK, WiFi.localIP().toString().c_str());

  // https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/system/system_time.html

  /* set a callback function on ntp sync */
  sntp_set_time_sync_notification_cb(print_time);

  /* sync the clock with ntp */
  configTzTime(TIMEZONE, NTP_POOL);
}

void loop() {
  //struct timeval tv_now;
  //gettimeofday(&tv_now, NULL);
  //print_time(&tv_now);
  delay(10);
}