int ledPin = LED_BUILTIN;
int buttonPin = 2; // Button connected to D2 and GND

unsigned long lowTime = millis();
unsigned long highTime = millis();
unsigned long difference = 0;
bool active = false;

void setup() {
  pinMode(ledPin, OUTPUT);
  pinMode(buttonPin, INPUT_PULLUP);
  Serial.begin(9600);
  Serial.println("Starting button test");
}

void loop() {
  int val = digitalRead(buttonPin);
  
  if (val == LOW && active == false) {
    lowTime = millis();
    difference = lowTime - highTime;
    Serial.print("LOW ");
    Serial.println(difference);
    active = true;
  }
  else if (val == HIGH && active == true) {
    highTime = millis();
    difference = highTime - lowTime;
    Serial.print("HIGH ");
    Serial.println(difference);
    active = false;
  }
}