Set<Date> times = new TreeSet<>();
Date date1 = new Date(1555587156974L);
Date date2 = new Date(1555588156974L);
Date date3 = new Date(1555589156974L);
times.add(date3);
times.add(date1);
times.add(date2);
SimpleDateFormat format = new SimpleDateFormat("hh:mm");
String timesString = times.stream().map(date -> format.format(date) + ";").collect(Collectors.joining());