I previously wrote about the off-by-one-hour problem that oudated timezone data files could cause false conversion to local time, and warned that certain programs might assume /etc/localtime being copied or symlinked from tzdata files in /usr/share/zoneinfo. This followup post demonstrates such an assumption and explores how Java applications decide their timezones, which was filed as a bug though.

Due to limited network access and too much old operating system, we applied a quick fix of by replacing obsolete only /etc/localtime with one from an up-to-date operating system. Then date(1) worked as expected, however, some guy reported that his Java program was logging in GMT time when restarted across the quick fix. Although it is a complex application, the defect could be reproduced by several lines of code. I would call this a good example of precise and reliable reproduction of defects, and the bad habbit of complaining in a complex context should be avoided completely.

The problem

Here is the source code reproducing the defect.

import java.util.Date;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
public class Test {
   public static void main(String args[]) {
      Date now = new Date();
      DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss, SSS/zzz");

It is able to produce correct localtime before quick fix even date(1) is not. After the quick fix is applied, date(1) output is OK, while the test program prints unexpected timezone. If both /etc/localtime and the one in /usr/share/zoneinfo are updated to correct version, both work with good output.

$ date
Wed Oct 31 19:28:12 PST 2012
$ java Test
2012-10-31 20:14:28,329/PDT

$ date
Wed Oct 31 20:28:47 PDT 2012
$ java Test
2012-11-01 03:49:28,902/GMT

$ date
Wed Oct 31 20:29:44 PDT 2012
$ java Test
2012-10-31 20:47:29,601/PDT


Time to explore timezone in the Java world.

Although the preliminary impression was Java maintains its own timezone database regardless of the host operating system, it confused me a little why Java fell back to GMT. Let’s stop guessing and dig into code.

TIPS: For quick source code access, visit links like java.util.Date, otherwise, the source bundle could be retrieved from official site.

Actually a Date object constructed with empty argument list simply captures epoch time with System.currentTimeMillis(), so that line of code is irrelavant. SimpleDateFormat(pattern) is a shortcut of SimpleDateFormat(pattern, Locale.getDefault()). Locale.getDefault() is singleton access to an object created with Locale(language,country,variant) which is simple encapsulation around string varriables, named language, country and variant, guessed out from system property user.language, user.region, user.country and user.variant. The default Locale essentially could be represented by something like en_US, and obviously is irrelavant.

So the hard parts probably lie in the overloaded constructor

491    public SimpleDateFormat(String pattern, Locale locale)
492    {
493        if (pattern == null || locale == null) {
494            throw new NullPointerException();
495        }
497        initializeCalendar(locale);
498        this.pattern = pattern;
499        this.formatData = DateFormatSymbols.getInstance(locale);
500        this.locale = locale;
501        initialize(locale);
502    }

The magic could be in line 497, 499 or 501.

DataFormatSymbols is some mapping between numeric values and string representations, so line 499 is irrelavant. Let’s take a look at line 497 which in turn calls Calendar.getInstance(TimeZone.getDefault(), loc). Now you might get hit by strong feelings that TimeZone.getDefault() is the point, and create another small program to verify it.

public class Tz {
   public static void main(String args[]) {

It actually clones an object created by private method setDefaultZone(), which determines the zoneID by system property user.timezone or

547        // if the time zone ID is not set (yet), perform the
548        // platform to Java time zone ID mapping.
549        if (zoneID == null || zoneID.equals("")) { 
550            String country = AccessController.doPrivileged(
551                    new GetPropertyAction("user.country"));
552            String javaHome = AccessController.doPrivileged(
553                    new GetPropertyAction("java.home"));
554            try {
555                zoneID = getSystemTimeZoneID(javaHome, country);
556                if (zoneID == null) {
557                    zoneID = GMT_ID;
558                }
559            } catch (NullPointerException e) {
560                zoneID = GMT_ID;
561            }
562        }
564        // Get the time zone for zoneID. But not fall back to
565        // "GMT" here.
566        tz = getTimeZone(zoneID, false);
568        if (tz == null) {
569            // If the given zone ID is unknown in Java, try to
570            // get the GMT-offset-based time zone ID,
571            // a.k.a. custom time zone ID (e.g., "GMT-08:00").
572            String gmtOffsetID = getSystemGMTOffsetID();
573            if (gmtOffsetID != null) {
574                zoneID = gmtOffsetID;
575            }
576            tz = getTimeZone(zoneID, true);
577        }

Bad news is getSystemTimeZoneID(javaHome, country) is native, good news is the comments are full of hits. Anyway, we meet what was noticed earlier, that Java produces right local times with explicit -Duser.timezone=America/Los_Angeles argument. Additionally we can guess that the native method should fail to get a zoneID in the environment where /etc/localtime is not from /usr/share/zoneinfo, resulting the program fall back to GMT as the last resort. Let’s trace the program to verify the point.

strace -o test.trace -f java Test 

One thing to note is that you should strace(1) with -f option to capture the Java program’s complete interaction with the system. Try that command before quick fix, after quick fix, then after good practice. Examine the trace file, and you will have interesting findings that

  • with alien /etc/localtime, Java searches through /usr/share/zoneinfo and gets no match. It fall back to take GMT as its timezone.
  • with /etc/localtime from /usr/share/zoneinfo, Java is able to determine its timezone from zone file name by either following the symlink, or traversing the directory to stat each tzdata file and stopping at the first match. It is irrelavant whether the content of the matched tzdata file is update or not.
  • with explicit timezone, no need to guess.

After having determined the name of its timezone, a Java program looks up with the mapping between system timezone and Java timezone, /usr/java/jdk1.6.0_30/jre/lib/zi/ZoneInfoMappings and locates the timezone database file of the Java world such as /usr/java/jdk1.6.0_30/jre/lib/zi/GMT which is of different format from tzfile(5) though. Java then uses its own interpretation of timezones, and is independent on operating system tzdata files.


Reproduce bugs with simple code, and try to reduce the context as much as possible. When you reduce to its limit, you understand the problem.

Update your operating system, at least essential parts.