You need to compute the difference between two dates.

Convert to Date objects if necessary, call their getTime( ) methods, and subtract. Format the result yourself.

There is no general mechanism in the API for computing the difference between two dates. This is surprising, given how often it comes up in some types of commercial data processing. However, it’s fairly simple to implement this yourself:

import java.util.*;

/** DateDiff -- compute the difference between two dates.
 */
public class DateDiff {
    public static void main(String[] av) {
        /** The ending date. This value
         * doubles as a Y2K countdown time.
         */
        Date d1 = new GregorianCalendar(1999,11,31,23,59).getTime(  );

        /** Today's date */
        Date d2 = new Date(  );

        // Get msec from each, and subtract.
        long diff = d2.getTime() - d1.getTime(  );

        System.out.println("Difference between " + d2 + "
" +
            "	and Y2K is " +
            (diff / (1000*60*60*24)) +
            " days.");
    }
}

Of course, I’m doing the final editing on this chapter long after the Y2K turnover, so it should print a positive value, and it does:

> java DateDiff
Difference between Fri Nov 03 19:24:24 EST 2000
        and Y2K is -307 days. 
>

You saw Calendar’s add( ) method in Section 6.8, but that only adds to the day, month, or year (or any other field) in the Calendar object; it does not add two Calendar dates together.