The need to provide compareTo

If a class is for a type where there is a natural order for the values, you ought to provide a method to find out whether one value is less than another.
Some parts of the Collections API (e.g., TreeSet and TreeMap) work better if your class implements the Comparable interface,

This interface is simply:

public interface Comparable
{
   public int compareTo(Object pObject);
}

To implement Comparable, Date can be changed to:

public class Date implements Comparable
{
   ...
   public int compareTo(final Object pObject)
   {
      final Date tDate = (Date)pObject;
      int tResult = iYear - tDate.iYear;
      if (tResult==0)
      {
         tResult = iMonth - tDate.iMonth;
         if (tResult==0)
         {
            tResult = iDay - tDate.iDay;
         }
      }
      return tResult;
   }
}

Comparable became part of Java when the Java 2 platform was released in December 1998.