Java code review

import java.util.Date; import java.util.concurrent.TimeUnit; public class Calendar { private static Calendar self; private static java.util.Calendar cal; private Calendar() { cal = java.util.Calendar.getInstance(); } public static Calendar getInstance() { if (self == null) { self = new Calendar(); } return self; } public void incrementDate(int days) { cal.add(java.util.Calendar.DATE, days); } public synchronized void setDate(Date date) { try { cal.setTime(date); cal.set(java.util.Calendar.HOUR_OF_DAY, 0); cal.set(java.util.Calendar.MINUTE, 0); cal.set(java.util.Calendar.SECOND, 0); cal.set(java.util.Calendar.MILLISECOND, 0); } catch (Exception e) { throw new RuntimeException(e); } } public synchronized Date Date() { try { cal.set(java.util.Calendar.HOUR_OF_DAY, 0); cal.set(java.util.Calendar.MINUTE, 0); cal.set(java.util.Calendar.SECOND, 0); cal.set(java.util.Calendar.MILLISECOND, 0); return cal.getTime(); } catch (Exception e) { throw new RuntimeException(e); } } public synchronized Date getDueDate(int loanPeriod) { Date now = Date(); cal.add(java.util.Calendar.DATE, loanPeriod); Date dueDate = cal.getTime(); cal.setTime(now); return dueDate; } public synchronized long getDaysDifference(Date targetDate) { long diffMillis = Date().getTime() - targetDate.getTime(); long diffDays = TimeUnit.DAYS.convert(diffMillis, TimeUnit.MILLISECONDS); return diffDays; } }