Oh sorry, in my last code, I observed an hour value incrementation when dst is active, because I omitted to define correctly the tm_isdst parameter.
Then I added an instruction in my getCustomTime function :
void getCustomTime(int year, int month, int day, int hour, int minute, int second, tm *timePtr)
{
timePtr->tm_year = year – 1900;
timePtr->tm_mon = month-1;
timePtr->tm_mday = day;
timePtr->tm_hour = hour;
timePtr->tm_min = minute;
timePtr->tm_sec = second;
timePtr->tm_isdst = 0;
time_t t = mktime(timePtr);
memcpy(timePtr, localtime(&t), sizeof(tm));
if (timePtr->tm_isdst==1) timePtr->tm_hour–;
}
In your code, the tm_isdst field is a parameter of the printLocalTime() function ; so no problem ! Except that it’s not obvious to know what is its correct value…
I would also reprecise that I don’t want to modify the RTC value : just to obtain a custom POSIX time in the “YYYY-MM-DDT00:00:00zzzzzz” format, because required in the RTE’s API Tempo Like Supply Contract.

You will find below the complete code for PlatformIO (main.cpp).
I just changed the name of the function to get a custom time : getCustomTime (analog to getLocalTime)
In comments, the suggested code by Random Nerd tutorial, completed to restore the RTC current time.
Thanks to say if may proposition is correct.
Best regards.
Jac
NB : in Paris, the time offset changed on the 31/03/2024 : +1h because summer time.

// Time – main.cpp

/***********************************************************************************

Objective
Time utilities for ESP32, that includes TZ and DST adjustments.
Get the POSIX style TZ format string from https://github.com/nayarsystems/posix_tz_db/blob/master/zones.csv
Inspired from Hardy Maxa and Random Nerd tutorials.

Output
IP=192.168.1.19 RSSI=-76
Current date-time : 2024-10-21 09:14:06+0200
Custom date-time : 2024-03-31 00:00:00+0100
Custom date-time : 2024-04-01 01:00:00+0200

References
– Wifi :
. https://randomnerdtutorials.com/esp32-useful-wi-fi-functions-arduino/
– Current time :
. https://randomnerdtutorials.com/esp32-date-time-ntp-client-server-arduino/
. https://randomnerdtutorials.com/esp32-ntp-timezones-daylight-saving/
. sourceware.org/newlib/libc.html#Timefns
. cplusplus.com/reference/ctime/
. github.com/espressif/arduino-esp32/blob/master/cores/esp32/esp32-hal-time.c#L47

/***********************************************************************************
Libraries and types
***********************************************************************************/

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

/***********************************************************************************
Constants
***********************************************************************************/

// Local network access point
const char *SSID = “Livebox-486c”;
const char *PWD = “FAAD971F372AA5CCE13D25969E”;

// NTP server (=>UTC time) and Time zone
const char* NTP_SERVER = “pool.ntp.org”; // Server address (or “ntp.obspm.fr”, “ntp.unice.fr”, …)
const char* TIME_ZONE = “CET-1CEST,M3.5.0,M10.5.0/3”; // Europe/Paris time zone

/***********************************************************************************
Tool functions
***********************************************************************************/

void initTime(const char *timeZone)
{
struct tm t;
configTime(0, 0, NTP_SERVER);
while (!getLocalTime(&t)); // Here use NTP server
setenv(“TZ”, timeZone, 1);
tzset();
}

void getCustomTime(int year, int month, int day, int hour, int minute, int second, tm timePtr)
{
timePtr->tm_year = year – 1900;
timePtr->tm_mon = month-1;
timePtr->tm_mday = day;
timePtr->tm_hour = hour;
timePtr->tm_min = minute;
timePtr->tm_sec = second;
timePtr->tm_isdst = 0;
time_t t = mktime(timePtr);
memcpy(timePtr, localtime(&t), sizeof(tm));
/
tm savTime;
getLocalTime(&savTime);
struct timeval now = {.tv_sec = t};
settimeofday(&now, NULL);
getLocalTime(timePtr);
t = mktime(&savTime);
now = {.tv_sec = t};
settimeofday(&now, NULL);
*/
}

/***********************************************************************************
setup and loop functions
***********************************************************************************/

void setup()
{
// Open serial port
Serial.begin(115200);
while (!Serial) {}

// Connect to the Wifi access point
WiFi.begin(SSID, PWD);
while (WiFi.status() != WL_CONNECTED); // delay(500);
Serial.printf(“IP=%s RSSI=%d\n”, WiFi.localIP().toString(), WiFi.RSSI());

// Init time
initTime(TIME_ZONE);

// Current date-time using RTC
struct tm time;
char buf[30];
getLocalTime(&time);
strftime(buf, sizeof(buf), “%Y-%m-%d %H:%M:%S%z\n”, &time);
Serial.printf(“Current date-time : %s”, buf);

// Custom date-time when offset changes
getCustomTime(2024, 3, 31, 0, 0, 0, &time);
strftime(buf, sizeof(buf), “%Y-%m-%d %H:%M:%S%z\n”, &time);
Serial.printf(“Custom date-time : %s”, buf);
getCustomTime(2024, 4, 1, 0, 0, 0, &time);
strftime(buf, sizeof(buf), “%Y-%m-%d %H:%M:%S%z\n”, &time);
Serial.printf(“Custom date-time : %s”, buf);
}

void loop()
{
}

Please do, Thank you

Yes.
This is only compatible with ESP32.
Regards.
Rui

i tried running this tutorial for ESp8266 and i get a compilation error “error: no matching function for call to ‘println(tm*, const char [34])'” on line :
Serial.println(&timeinfo, “%A, %B %d %Y %H:%M:%S zone %Z %z “);

Is that because of ESp8266?

Thanks

Hello,

for europe use this timeserver:
String ntp = “de.pool.ntp.org”;
and
configTzTime(“CET-1CEST,M3.5.0/03,M10.5.0/03”, ntp.c_str());

then it works perfect with summer and winter time

Regards