The Java JDK doesn't have methods for obtaining the time difference between two Date objects and this is often what people want to know. It's reasonably easy to perform this calculation but a 'gotcha' that has to be avoided is offsets due to timezone. Below is one way to do it, using Universal Time (UTC) so that the daylight -saving offset problem can be obviated. The source is available HERE. Addendum: you can see in the second code extract below, the solution with the java.time package (introduced in Java 1.8) is a lot simpler.

import java.util.Calendar;
import java.util.Date;
import java.util.TimeZone;

public class TimeDiff {
/**
* (For testing purposes)
*
*/
public static void main(String[] args) {
Date d1 = new Date();
try { Thread.sleep(750); } catch(InterruptedException e) { /* ignore */ }
Date d0 = new Date(System.currentTimeMillis() - (1000*60*60*24*3)); // About 3 days ago
long[] diff = TimeDiff.getTimeDifference(d0, d1);

System.out.printf("Time difference is %d day(s), %d hour(s), %d minute(s), %d second(s) and %d millisecond(s)\n",
diff[0], diff[1], diff[2], diff[3], diff[4]);
System.out.printf("Just the number of days = %d\n",
TimeDiff.getTimeDifference(d0, d1, TimeDiff.TimeField.DAY));
}

/**
* Calculate the absolute difference between two Date without
* regard for time offsets
*
* @param d1 Date one
* @param d2 Date two
* @param field The field we're interested in out of
* day, hour, minute, second, millisecond
*
* @return The value of the required field
*/
public static long getTimeDifference(Date d1, Date d2, TimeField field) {
return TimeDiff.getTimeDifference(d1, d2)[field.ordinal()];
}

/**
* Calculate the absolute difference between two Date without
* regard for time offsets
*
* @param d1 Date one
* @param d2 Date two
* @return The fields day, hour, minute, second and millisecond
*/
public static long[] getTimeDifference(Date d1, Date d2) {
long[] result = new long[5];
Calendar cal = Calendar.getInstance();
cal.setTimeZone(TimeZone.getTimeZone("UTC"));
cal.setTime(d1);

long t1 = cal.getTimeInMillis();
cal.setTime(d2);

long diff = Math.abs(cal.getTimeInMillis() - t1);
final int ONE_DAY = 1000 * 60 * 60 * 24;
final int ONE_HOUR = ONE_DAY / 24;
final int ONE_MINUTE = ONE_HOUR / 60;
final int ONE_SECOND = ONE_MINUTE / 60;

long d = diff / ONE_DAY;
diff %= ONE_DAY;

long h = diff / ONE_HOUR;
diff %= ONE_HOUR;

long m = diff / ONE_MINUTE;
diff %= ONE_MINUTE;

long s = diff / ONE_SECOND;
long ms = diff % ONE_SECOND;
result[0] = d;
result[1] = h;
result[2] = m;
result[3] = s;
result[4] = ms;

return result;
}

public static void printDiffs(long[] diffs) {
System.out.printf("Days:         %3d\n", diffs[0]);
System.out.printf("Hours:        %3d\n", diffs[1]);
System.out.printf("Minutes:      %3d\n", diffs[2]);
System.out.printf("Seconds:      %3d\n", diffs[3]);
System.out.printf("Milliseconds: %3d\n", diffs[4]);
}

public static enum TimeField {DAY,
HOUR,
MINUTE,
SECOND,
MILLISECOND;
}
}

The code after the advent of the new java.time package makes this all a lot simpler:

import java.time.LocalDate;
import java.time.Month;
import java.time.temporal.ChronoUnit;

public class HowLongTilXmas {
public static void main(String[] args) throws Exception {
LocalDate christmas = LocalDate.now().withMonth(Month.DECEMBER.getValue()).withDayOfMonth(25);
System.out.printf("How many days until Christmas? Answer: %d%n", ChronoUnit.DAYS.between(LocalDate.now(), christmas));
}
}