Difficulty creating a function from a varriable

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1

Hi- I am very new to programming, and am stuck. I need to read the state of a button press, and if it's held longer than 500ms, I would like to turn an LED on. If it's < 500ms, the loop should continue. I"m frustrated because this seems like it should be easy, but my code isn't working:

//GSwitch timer test

const int ledPin = 11;
const int startPin = 10;
long startTime;
unsigned long duration;



void setup()
{
  pinMode(13,OUTPUT);
  digitalWrite(13,LOW);
  pinMode(startPin, INPUT);
  digitalWrite(startPin, HIGH);
  pinMode(ledPin, OUTPUT);
 
  Serial.begin(9600);
}

void loop()
{
  if(digitalRead(startPin) == LOW)
  {
     // here if the switch is pressed
     startTime = millis();
     while(digitalRead(startPin) == LOW)
        ; // wait while the switch is still pressed
    duration = millis() - startTime;
  }
     if(duration >= 500);
     {
       digitalWrite (ledPin,HIGH);
     Serial.println(duration);
  }

}

When I watch in the serial monitor, I can see the button presses registering and reasonable times are being reported out, but within half a second of resetting, my LED pin goes high (#11), regardless of button activity.

2 
if(duration >= 500);

What's that semicolon doing there?