ESP32 Uptime example

uptime.ino

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

void loop() {
  char buf[50];
  int64_t upTimeUS = esp_timer_get_time(); // in microseconds
  int64_t seconds = upTimeUS/1000000;
  uint32_t days = (uint32_t)seconds/86400;
  uint32_t hr=(uint32_t)seconds % 86400 /  3600;
  uint32_t min=(uint32_t)seconds %  3600 / 60;
  uint32_t sec=(uint32_t)seconds % 60;
  snprintf (buf,sizeof(buf),"%dd %d:%02d:%02d", days, hr, min, sec);
  String upTime = String(buf);
  Serial.println(upTime);
}