The reason behind this is the daylight saving time conventions. An old JVM won't necessarily have the same daylight saving time for a given TimeZone than a latest JVM, and therefore will interpret the date differently.
Here is the output of a very simple program on 2 different JVMs. The number is the long number used to represent the date. I only use SimpleDateFormat with different TimeZone:
JVM 1.5.0_12
loading date=Sat Jul 18 06:59:36 CEST 2009, number=1247893176505
using formatter in EST: 7/17/09 11:59 PM
using formatter in Asia/Singapore: 7/18/09 12:59 PM
JVM 1.5.0_20
loading date=Sat Jul 18 06:59:36 CEST 2009, number=1247893176505
using formatter in EST: 7/18/09 12:59 AM
using formatter in Asia/Singapore: 7/18/09 12:59 PM
The source code:
What does this mean? This means that if you entered in a GUI in the first JVM a particular date & time using EST time zone. This will change when you read back in the second JVM.
public class DateBug {
private static String FILE_NAME = "datebug.txt";
public static void load() throws IOException {
FileReader fr = new FileReader(FILE_NAME);
BufferedReader br = new BufferedReader(fr);
String l = br.readLine();
br.close();
long time = new Long(l);
Date d = new Date(time);
System.out.println("loading date="+d+", number="+d.getTime());
SimpleDateFormat formatter = new SimpleDateFormat();
formatter.setTimeZone(TimeZone.getTimeZone("EST"));
System.out.println("using formatter in EST: "+formatter.format(d));
formatter.setTimeZone(TimeZone.getTimeZone("Asia/Singapore"));
System.out.println("using formatter in Asia/Singapore: "+formatter.format(d));
}
public static void saveNew() throws IOException {
Calendar c = Calendar.getInstance(TimeZone.getTimeZone("EST"));
c.set(2009, 06, 17, 23, 59);
Date d = c.getTime();
System.out.println("saving date="+d+", number="+d.getTime());
SimpleDateFormat formatter = new SimpleDateFormat();
formatter.setTimeZone(TimeZone.getTimeZone("EST"));
System.out.println("using formatter in EST: "+formatter.format(d));
formatter.setTimeZone(TimeZone.getTimeZone("Asia/Singapore"));
System.out.println("using formatter in Asia/Singapore: "+formatter.format(d));
FileWriter fw = new FileWriter(FILE_NAME);
PrintWriter pw = new PrintWriter(fw);
pw.println(d.getTime());
pw.close();
}
public static void main(String[] args) throws IOException {
System.out.println("JVM "+System.getProperty("java.version"));
if (args.length == 1) {
if (args[0].equals("save")) {
saveNew();
}
} else {
load();
}
}
This suggests that if you want to keep the same dates, you are better off saving in UTC where daylight saving time is not used and trick DateFormat. But I have to say this looks quite ugly.
