int pin = A0;
int led = 12;
void setup()
{
  pinMode(pin, INPUT_PULLUP);
  pinMode(led, OUTPUT);
}

long offAt = 0;

void loop()
{
  if( (digitalRead(led) == LOW ) && (digitalRead(pin) == LOW) ) //if LED is off  and button is pressed [low because it has pullup resistor]
  { 
    digitalWrite(led, HIGH);
    offAt = millis() + 5000; //store var of now + 5 seconds
  }
  
  
  if(digitalRead(led) == HIGH) //if led is on
  {
      if(millis() >= offAt) //see if it's time to turn off LED
      {
         digitalWrite(led, LOW); //it's time. this also re-enables the button
      }
  }
}