Send LED HIGH after "x" seconds

Using Arduino LEDs and Multiplexing
4

I don't know which is the active state of a PIR sensor .
My assumption, for this sketch, was that it would go Low.
You can change that easily.

Anyway, once the 5 consecutive second threshold is crossed, LED 13 goes On.
And it stays on till the Reset button is pushed or you include a separate reset option

//  trippoint5sec
//
//  wait for 5 seconds
//  of constant detection
//

unsigned long refTime;
unsigned long trippoint;
const long threshold = 5000;
byte DET;
const byte pirPin = 8;
const byte ledPin = 13;

void setup ()
{
  //pirPin is INPUT by default, use ExtPullup
  pinMode (ledPin,OUTPUT);
  digitalWrite (ledPin,LOW);
}

void loop ()
{
  DET = digitalRead(pirPin);
  if (DET == 1)  // inactive
  {
    refTime = millis();
    trippoint = refTime;
  }
  else   // Active Low
  {
    pir_active();
  }
}

void pir_active ()
{
  trippoint = millis();
  if ((trippoint-refTime) > threshold)
  {
    digitalWrite(ledPin,HIGH);
  }
}