To manipulate dates (and times) you should use the Calendar class:
// calendar set to current date and time
Calendar cal = Calendar.getInstance();
// Add ten hours to the current date/time
cal.add(Calendar.HOUR, 10);
// Get back the date in the calendar so we
// can pass it around our application
Date date = calendar.getTime();
// Print if, for example
SimpleDateFormat format = new SimpleDateFormat("dd/MM/yyyy hh:mm:ss");
System.out.println(format.format(date));
Hope this helps.