Timing an interrupt driven button

button.ino

#define LONGPRESS_TIME 1000 // 1 second

volatile int trackButton = 0;
volatile unsigned long buttonChangeMillis = 0;

void setup() {
    attachInterrupt(0, buttonPressed, CHANGE);
}

void buttonPressed()
{
  if (trackButton == 0) {
    buttonChangeMillis = millis();
    trackButton = 1;
  } else {
    trackButton = 0;
    unsigned long now = millis();
    if ((now - buttonChangeMillis) <= LONGPRESS_TIME) {
      // Short press
    } else {
      // Long press
    }
  }
}