calculador de tiempo con rfid que cuente cuando pasa la tarjeta y pare de contar cuando vuelva a pasar la tarjeta rfid

#include <SPI.h>
#include <MFRC522.h>

#define RST_PIN 9            // Configurable, see typical pin layout above
#define SS_PIN 10           // Configurable, see typical pin layout above

MFRC522 mfrc522(SS_PIN, RST_PIN);  // Create MFRC522 instance

void setup() {
  Serial.begin(9600);     // Initialize serial communications with the PC
  while (!Serial);        // Do nothing if no serial port is opened (added for Arduinos based on ATMEGA32U4)
  SPI.begin();            // Init SPI bus
  mfrc522.PCD_Init();     // Init MFRC522 card
  Serial.println("Approximate your card to the reader...");
  Serial.println();
}

void loop() {
  // Look for new cards
  if ( ! mfrc522.PICC_IsNewCardPresent()) {
    return;
  }

  // Select one of the cards
  if ( ! mfrc522.PIC