void setup() {
  Serial.begin(115200);
  Serial.println("Setup-Start");

  unsigned long StartTime = 1; 
  Serial.print("StartTime = ");
  Serial.println(StartTime);

  unsigned long StopTime = 5; // simulating beeing just after the rollover
  Serial.print("StopTime = ");
  Serial.println(StopTime);

  unsigned long TimeDifference = StopTime - StartTime; 
  Serial.println("TimeDifference = StopTime - StartTime");

  Serial.print("TimeDifference = ");
  Serial.print(StopTime);
  Serial.print(" - ");
  Serial.print(StartTime);
  Serial.print(" = ");
  Serial.println(TimeDifference);

  StopTime = 1; // simulating beeing just after the rollover
  Serial.print("StopTime = ");
  Serial.println(StopTime);
  
  StartTime = StopTime - TimeDifference; // result is a very large number simulating beeing near of a rollover
  Serial.println("StartTime = StopTime - TimeDifference;");
  Serial.print("StartTime = ");
  Serial.print(StopTime);
  Serial.print(" - ");
  Serial.print(TimeDifference);
  Serial.print(" = ");
  Serial.print(StartTime);
  Serial.println(" very big number simulating millis() short before rollover");
  Serial.println();
  Serial.println();
  Serial.println();

  TimeDifference = StopTime - StartTime; // still a small number because of unsigned integer-math
  Serial.println("TimeDifference = StopTime - StartTime");
  Serial.println("TimeDifference = small number (after rollover) - big number before rollover");
  Serial.print("TimeDifference = ");
  Serial.print(StopTime);
  Serial.print(" - ");
  Serial.print(StartTime);
  Serial.print(" = ");
  Serial.print(TimeDifference);
  Serial.println(" still the correct result ");
  Serial.print(" same difference ");
  Serial.println(TimeDifference);
}

void loop() {
}