unsigned long blnkTme;
int ledState = 1;

void setup() {

Serial.begin(9600);
Serial.println(blnkTme);
digitalWrite(LED_BUILTIN, HIGH);
}

void loop() {

  Serial.println(millis()); 
if (millis() - blnkTme > 1000) {
  blnkTme = millis();
  if (ledState == 1) {
  digitalWrite(LED_BUILTIN, LOW);
  ledState = 0;
  }
  else {
  digitalWrite(LED_BUILTIN, HIGH);
  ledState = 1;
  }
}
}