### Question:
DateUtils { public static String format2Readable(long ms) { long days = ms / (1000 * 60 * 60 * 24); long hours = (ms % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60); long minutes = (ms % (1000 * 60 * 60)) / (1000 * 60); long seconds = (ms % (1000 * 60)) / 1000; return String.format("%02d %02d:%02d:%02d", days, hours, minutes, seconds); } private  DateUtils(); static String getCurrentTime(); static String getCurrentTime(String format); static String format(Date date, String format); static String format(LocalDateTime localDateTime, String format); static String dateToString(Date date); static Date parse(String date, String format); static Date stringToDate(String str); static long differSec(Date d1, Date d2); static long differMs(Date d1, Date d2); static long diffHours(Date d1, Date d2); static long diffMin(Date d1, Date d2); static Date getSomeDay(Date date, int day); static int getHourIndex(Date date); static boolean compare(Date future, Date old); static Date getScheduleDate(String schedule); static String format2Readable(long ms); static Date getMonday(Date date); static Date getSunday(Date date); static Date getFirstDayOfMonth(Date date); static Date getSomeHourOfDay(Date date, int offsetHour); static Date getLastDayOfMonth(Date date); static Date getStartOfDay(Date inputDay); static Date getEndOfDay(Date inputDay); static Date getStartOfHour(Date inputDay); static Date getEndOfHour(Date inputDay); static Date getCurrentDate(); static Date add(final Date date, final int calendarField, final int amount); static long getRemainTime(Date baseTime, long intervalSeconds); static String getCurrentTimeStamp();  }### Answer:
@Test public void format2Readable() throws ParseException { SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); String start = "2015-12-21 18:00:36"; Date startDate = sdf.parse(start); String end = "2015-12-23 03:23:44"; Date endDate = sdf.parse(end); String readableDate = DateUtils.format2Readable(endDate.getTime() - startDate.getTime()); Assert.assertEquals("01 09:23:08", readableDate); }