const byte LED_Pin = 13;
unsigned long BlinkTimer;
boolean LED_State = false;

void setup() {
  // put your setup code here, to run once:
  pinMode(LED_Pin,OUTPUT);
}


boolean TimePeriodIsOver (unsigned long &periodStartTime, unsigned long TimePeriod) {
  unsigned long currentMillis  = millis();  
  if ( currentMillis - periodStartTime >= TimePeriod )
  {
    periodStartTime = currentMillis; // set new expireTime
    return true;                // more time than TimePeriod) has elapsed since last time if-condition was true
  } 
  else return false;            // not expired
}


void loop() {
  // put your main code here, to run repeatedly:
  if (TimePeriodIsOver (BlinkTimer,500) ) {
    if (LED_State == false) {
      LED_State = true;
      digitalWrite(LED_Pin,HIGH);
    }
    else {
      LED_State = false;
      digitalWrite(LED_Pin,LOW);
    }    
  }
}