Java Data Type - 如何得到日期之间的差异

import java.util.Date;
import java.util.GregorianCalendar;

public class Main {
  public static void main(String[] av) {
    Date d1 = new GregorianCalendar(2000, 11, 31, 23, 59).getTime();

    Date today = new Date();

    long diff = today.getTime() - d1.getTime();

    System.out.println("The 21st century (up to " + today + ") is "
        + (diff / (1000 * 60 * 60 * 24)) + " days old.");
  }
}