const int ledPin = 13;
int delayTime = 500;

void setup() {
 pinMode(ledPin, OUTPUT);
}

void loop() {
  digitalWrite(ledPin, HIGH);
  delay(delayTime);
  digitalWrite(ledPin, LOW);
  delay(delayTime);
  delayTime += 100;
  if (delayTime > 1000) {
     delayTime = 500;
  }
}