//define global variables
unsigned long previousTimeSerialPrint;
unsigned long timeNow;
unsigned long time_delay = 500; //keep as unsigned long 
//so that the data types are the same

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  previousTimeSerialPrint = millis();
} 

void loop() {
  // put your main code here, to run repeatedly:
  timeNow = millis();
  if (timeNow - previousTimeSerialPrint > time_delay){
    Serial.println("Hello World");
    previousTimeSerialPrint = millis(); //resets the time to 0
  }
}

/*
or you could set previousTimeSerialPrint = timeNow, 
this will give the same result
*/