sntp based time starts running backwards

I used the example of extras/sntp and then, based on a timer that triggers every second, collect a time_t=time(NULL) when 60 triggers have passed

This should give a constant update of near 1 minute in reported time which is does... HOWEVER, after a certain time (not always the same), the clock reports backwards by 7:44:50 (not always the same, but within a minute yes). It then continues going backward for an amount of time (not always the same*) and at that 'moment in the past' is starts reporting again 1 minute up until it goes back by 7:44:xx again and then gets back in gear

*) for one session (uninterrupted by a restart) the bad-run-time was 115 minutes twice, but another session it is much more. Is this based on a bug? there seems to be some repeatable pattern so it suggests a code based issue.

Any help is appreciated,

relevant pieces of code below

EXTRA_COMPONENTS = extras/sntp 

#define SNTP_SERVERS "0.pool.ntp.org", "1.pool.ntp.org", "2.pool.ntp.org", "3.pool.ntp.org"
void time_task(void *argv) {
    while (sdk_wifi_station_get_connect_status() != STATION_GOT_IP) vTaskDelay(20); 
    const char *servers[] = {SNTP_SERVERS};
    sntp_set_update_delay(24*60*60000); //SNTP will request an update every 24 hours
    const struct timezone tz = {1*60, 0}; //Set GMT+1 zone, daylight savings off
    sntp_initialize(&tz);
    sntp_set_servers(servers, sizeof(servers) / sizeof(char*)); //Servers must be configured right after initialization
    time_t ts;
    do {ts = time(NULL);
        if (ts == ((time_t)-1)) vTaskDelay(10);
    } while (!(ts>1608567890)); //Mon Dec 21 17:24:50 CET 2020
    printf("TIME: %ds %u=%s\n", ((unsigned int)ts)%86400, (unsigned int) ts, ctime(&ts));
    vTaskDelete(NULL); //check if NTP keeps running without this task
}

void heater1(uint32_t seconds) {
    time_t ts = time(NULL);
    char timestring[26]; ctime_r(&ts,timestring); timestring[19]=0;
    printf("Heater1 @ %d: S1avg=%2.4f S2avg=%2.4f %s", (seconds+10)/60, S1avg, S2avg, timestring+4\n);
}

TimerHandle_t xTimer;
void vTimerCallback( TimerHandle_t xTimer ) {
    uint32_t seconds = ( uint32_t ) pvTimerGetTimerID( xTimer );
    vTimerSetTimerID( xTimer, (void*)seconds+1); //136 year to loop
    if (seconds%60==50) {
        heater1(seconds);
    }
}

user_init:
    xTaskCreate(time_task,"Time", 512, NULL, 1, NULL);
    xTimer=xTimerCreate( "Timer", 1000/portTICK_PERIOD_MS, pdTRUE, (void*)0, vTimerCallback);

and a few lines of the output

Heater1 @ 346: S1avg=21.8750 S2avg=20.4792 Dec 23 22:29:27
Heater1 @ 347: S1avg=21.8750 S2avg=20.4896 Dec 23 22:30:28
Heater1 @ 348: S1avg=21.8750 S2avg=20.4896 Dec 23 22:31:28
Heater1 @ 349: S1avg=21.8646 S2avg=20.5000 Dec 23 07:03:05 <=double skip
Heater1 @ 350: S1avg=21.8750 S2avg=20.5000 Dec 22 23:19:24
Heater1 @ 351: S1avg=21.8750 S2avg=20.4896 Dec 22 15:35:42
Heater1 @ 352: S1avg=21.8750 S2avg=20.4792 Dec 22 07:52:01
Heater1 @ 353: S1avg=21.8750 S2avg=20.4792 Dec 22 00:08:20

maybe a conflict in multiple places where time gets defined?

#include <stdio.h>
#include <espressif/esp_wifi.h>
#include <espressif/esp_sta.h>
#include <espressif/esp_system.h> //for timestamp report only
#include <esp/uart.h>
#include <esp8266.h>
#include <FreeRTOS.h>
#include <task.h>
#include <semphr.h>
#include <timers.h>
#include <homekit/homekit.h>
#include <homekit/characteristics.h>
#include <string.h>
#include "lwip/api.h"
#include <udplogger.h>
#include "ds18b20/ds18b20.h"
#include "i2s_dma/i2s_dma.h"
#include "math.h"
#include <sntp.h>

SuperHouse / esp-open-rtos

sntp based time starts running backwards #767