unsigned long prev_time =0;
unsigned long curr_time =0;
int period =1000;
int led_pin = 13;
void setup() {
Serial.begin(9600);
 pinMode (led_pin, OUTPUT); 
 prev_time = millis();
 
}

void loop() {
   curr_time =millis();
   if((curr_time-prev_time)> period){
    digitalWrite(led_pin,!digitalRead(13));
    prev_time = curr_time;
   }
  }