unsigned long timebegin; //as soon as you make the variable, the timer starts
// to make time a global variable, you must assign it a value

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  timebegin = millis();
} 

void loop() {
  // put your main code here, to run repeatedly:
  delay(500);
  unsigned long timenow = millis();
  unsigned long duration = timenow - timebegin;
  Serial.print("The duration is: "); 
  Serial.println(duration);
}

// what we have done here is measure the time of an
// action.