POTENTIOMETER SENSOR AND BUTTON

HELLO EVERYONE ,
I have this code and would like to add a button that I can interrupt the ignition of a LED before the set time,
the function of the code is to use a potentiometer, which activates a led me at (threshold = 400) by turning the potentiometer reaches the value and the
led me on, until you press the button the LED should enceder to arrive at the value, now if I press the button before time 15sec the LED is not activated, important is to be executed once mientra the arduino is energized.

unsigned long start = 0;
const int threshold = 400;
const int analogPin = A0;
int led = 6;


void setup() {

  start = millis(); 
  pinMode (led, OUTPUT);
  Serial.begin(9600);

}

void loop() {
  while (millis() - start <= 15000) {
    int analogValue = analogRead(analogPin);
    if (analogValue > threshold)
      digitalWrite(led, HIGH);
    else
      digitalWrite(led, LOW);
    // print the analog value:
    Serial.println(analogValue);

  } // fin del while
}

If you want a responsive program use IF rather than WHILE

Have a look at the code in Several Things at a Time for the usual way to use millis().

...R