char* buildTimeString(char * dest, size_t maxSize) {
  // here I use random values, obviously you would extract that from your time source like NTP
  int hour = random(0, 24);
  int min = random(0, 60);
  int sec = random(0, 60);;
  snprintf(dest, maxSize, "%02d:%02d:%02d", hour, min, sec);
  return dest;
}

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

void loop() {
  char tmpBuffer[9]; // we allocate space, enough for HH:MM:SS 8 chars + 1 trailing null
  Serial.println(buildTimeString(tmpBuffer, sizeof tmpBuffer));
  delay(1000);
}