Java Cookbook, Second Edition

Problem

You need to add or subtract a fixed amount to or from a date.

Solution

As we've seen, Date has a getTime( ) method that returns the number of seconds since the epoch as a long . To add or subtract, you just do arithmetic on this value. Here's a code example:

// DateAdd.java /** Today's date */ Date now = new Date( ); long t = now.getTime( ); t -= 700L*24*60*60*1000; Date then = new Date(t); System.out.println("Seven hundred days ago was " + then);

Discussion

A cleaner variant is to use the Calendar's add( ) method. There is no corresponding subtraction method; you just add a negative value to make time run backward:

import java.text.*; import java.util.*; /** DateCalAdd -- compute the difference between two dates. */ public class DateCalAdd { public static void main(String[] av) { /** Today's date */ Calendar now = Calendar.getInstance( ); /* Do "DateFormat" using "simple" format. */ SimpleDateFormat formatter = new SimpleDateFormat ("E yyyy/MM/dd 'at' hh:mm:ss a zzz"); System.out.println("It is now " + formatter.format(now.getTime( ))); now.add(Calendar.YEAR, - 2); System.out.println("Two years ago was " + formatter.format(now.getTime( ))); } }

Running this reports the current date and time, and the date and time two years ago:

> java DateCalAdd It is now Tue 2003/11/25 at 09:14:26 AM EST Two years ago was Sun 2001/11/25 at 09:14:26 AM EST

A method called roll( ) does not change "larger" fields. For example, rolling the month up from January 31, 2051 results in February 28, 2051.