In

public Date add(Date date, Duration duration) {
Date end = null;

Calendar calendar = BusinessCalendar.getCalendar();
calendar.setTime(date);
int hour = calendar.get(Calendar.HOUR_OF_DAY);
int minute = calendar.get(Calendar.MINUTE);

long dateMilliseconds = ((hour*60)+minute)*60*1000;
long dayPartEndMilleseconds = ((toHour*60)+toMinute)*60*1000;

change last two lines into:

long dateMilliseconds = ((hour*60L)+minute)*60*1000;
long dayPartEndMilleseconds = ((toHour*60L)+toMinute)*60*1000;