Why does a function inside void set up is repeating after some minutes

Question

I am trying to time sync my arduino hardware to NTP server. I have a `setSyncProvider(getNtpTime)´ function to do it and called inside the void setup. I have another function (mainFunction) inside the main loop, that calls every minute. I have added a function to display time inside this mainFunction so that time will update every minutes. Code works fine for first five minutes. Then it calls the getNtpTime again even though it is not inside the loop. Why is it happening?

time_t getNtpTime()
{
  while (Udp.parsePacket() > 0) ; // discard any previously received packets
  Serial.println("Transmit NTP Request");
  sendNTPpacket(timeServer);
  uint32_t beginWait = millis();
  while (millis() - beginWait < 1500) {
    int size = Udp.parsePacket();
    if (size >= NTP_PACKET_SIZE) {
      Serial.println("Receive NTP Response");
      Udp.read(packetBuffer, NTP_PACKET_SIZE);  // read packet into the buffer
      unsigned long secsSince1900;
      // convert four bytes starting at location 40 to a long integer
      secsSince1900 =  (unsigned long)packetBuffer[40] << 24;
      secsSince1900 |= (unsigned long)packetBuffer[41] << 16;
      secsSince1900 |= (unsigned long)packetBuffer[42] << 8;
      secsSince1900 |= (unsigned long)packetBuffer[43];
      return secsSince1900 - 2208988800UL + timeZone * SECS_PER_HOUR;
    }
  }
  Serial.println("No NTP Response :-(");
  return 0; // return 0 if unable to get the time
}

Answer 1

The setSyncProvider function of the TimeLib sets a function which will be called by the library every interval seconds. The interval is set with setSyncInterval. Default interval is 300 seconds.