#include <TimerOne.h>

#define LED_PIN 4

unsigned long int interval = 0-1;  // just to get biggest possible value
unsigned long int current_time = 0;
unsigned long int start_time = 0;

bool state = false;
bool previous_state = !state; // it need to different

void change_led_state(){
  state = !state;
  digitalWrite(LED_PIN, state);
}

void setup(){
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
}

void loop(){
    if(state != previous_state){  // if Timer1 was executed
      previous_state = state;
      current_time = millis();
      Serial.print("Interval was: ");
      Serial.println(current_time - start_time);
      Timer1.initialize(interval);
      Timer1.attachInterrupt(change_led_state);
      start_time = current_time;
    }
}