Well, it sounds like it would be a good idea to separate this into more appropriate types:
LocalTime userTime = ...; // Parse the user input
DateTime now = DateTime().now();
LocalDate today = now.toLocalDate();
LocalDate resultDate = now.toLocalTime().isBefore(userTime) ? today : today.plusDays(1);
DateTime result = resultDate.toDateTime(userTime, now.getTimeZone());