code: public static int getDayNum() {
          int daynum = 0;
          GregorianCalendar gd = new GregorianCalendar();
          Date dt = gd.getTime();
          GregorianCalendar gd1 = new GregorianCalendar(1900, 1, 1);
          Date dt1 = gd1.getTime();
          daynum = (int) ((dt.getTime() - dt1.getTime()) / (24 * 60 * 60 * 1000));
          return daynum;
      }
docs: 取得当前时间距离1900/1/1的天数
  
  @return 天数