TimeZone : Java Glossary

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What are Time Zones

In ordinary use, a time zone is a region of the earth that keeps the same winter time. They may or may not all flip to summer time at the same time and some parts may not flip at all.

TimeZone Names and Offsets

The reason that local time is different at different spots on the earth is a consequence of:

• The earth rotates on its axis.
• The earth rotates counter-clockwise when viewed looking down on the north pole.
• The earth rotates west to east.
• The sun rises in the east and sets in the west.
• In natural time, noon is when the sun is highest in the sky. This happens at one instant each day and at a different instant each day for each degree of latitude.
• People “back east” get up earlier and go to bed earlier.

You will see many different ways of specifying a time zone, including:

• Olson: America/Vancouver
• Posix: PST8PDT
• UTC relative UTC-8 UTC-7

Here is a list of available TimeZones:

If, TZ, the above TimeZone Java Applet does not work…

1. If Copy/Paste (Ctrl-C/Ctrl-V) do not work, you can turn them back on by modifying your java.policy file. This is not for the novice or faint of heart. instructions Your alternative is to download this program and run it without a browser.
2. Often problems can be fixed simply by clicking the reload button on your browser.
3. Make sure you have both JavaScript and Java enabled in your browser.
4. This Java Applet needs 32-bit (not 64-bit) Java 1.6 or later. For best results use the latest 1.7.0_02. If you have both 32 and 64-bit JVMs installed, in the Java Control Panel, configure your 32-bit java.exe as the user JVM and your 64-bit java.exe as the system JVM. You also need a recent browser.
5. It works under any operating system that supports Java e.g. W2K/XP/W2003/Vista/W7-32/W7-64/Linux/OSX
6. You should see the Applet above looking much like this screenshot. If you don’t, the following hints should help you get it working:
7. Especially if this Applet has worked before, try clearing the browser cache and rebooting.
8. To ensure your Java is up to date, check with Wassup. First, download it and run it as an application independent of your browser, then run it online as an Applet to add the complication of your browser.
9. If the above Applet does not work, check the Java console for error messages.
10. If the above Applet does not work, you might have better luck with the downloadable version available below.
11. If you are using Mac OS X and would like an improved Look and Feel, download the QuaQua look & feel from randelshofer.ch/quaqua. UnZip the contained quaqua.jar and install it in ~/Library/Java/Extensions or one of the other ext dirs.
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14. If you are using Microsoft Internet Explorer 9, makes sure the Java Plug-In SSV helper add-in is installed and enabled. If it is not, try reinstalling the Java JRE.
15. If you have Windows 7 64-bit and Internet Explorer 64-bit, in theory you can use 64-bit Java, but I never been able to get it to work.
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• Add column 1 in hours to UTC to get local standard time.
• Add column 2 in hours to UTC to get local daylight saving time.
• Use UTC when you want no time zone at all.
• Use Asia/Riyadh for Arabia Standard Time. Asia/Riyadh87, Asia/Riyadh88 and Asia/Riyadh89 are 3 hours and 7 minutes east of UTC. This is the offset used in 1987 to 1989. Actually it was 3 hours 7 minutes and 4 seconds to approximate solar time. Prior to 1950 they used 3:06:52. In the period 1951— 1986 and 1990 onward they used a simple 3 hour difference. In Islamic tradition, the day starts at sunset.
• Note how much many aliases there are for the same time zone. I speculate there are two reasons for this:
  1. To anticipate some geographical region adopting a quirky daylight saving rule in future. There would be no need adjust to a new split time zone to adopt the new rule. Only the tables built into Java would need to be adjusted.
  2. It makes it easier for people to find their own time zone. They need find only a nearby city without having to consult a map to find a distant city on the same longitude. Often they can find their own city directly.

Gotchas

• Timezone names never contain spaces. They use underscores, e. g. America/Los_Angeles. In Linux, make sure /etc/sysconfig/clock use official underscore names.
• Use UTC rather that GMT (Greenwich Mean Time) for the base time used internally. GMT is London civil time and has a DST (Daylight Saving Time) correction in summer. UTC huh not CUT? It was a weird compromise acronym half way between French and English. GMT stands for GMT.

See GNU Timezone classes that correct for historical changes and leap seconds.

You can find out the current time at any place on the globe at WorldTimeServer.com. It will tell you the offset from UTC, but not the time zone name. huh not CUT? It was a weird compromise acronym half way between French and English.

Unfortunately, you can’t extract the rules about when daylight savings go into effect from a TimeZone object. I suppose if you were patient you could deduce them by binary search on inDaylightTime. You could also study the code that is used to construct all the TimeZone objects in rt.jar, or the various locale jars and decompile and parse that to extract the information.

Here are the rules for the switch days in the EU. There are some algorithms to compute them.

Switch days are decided politically so can change erratically, especially in the USA, albeit with advance notice. In ancient Rome, one could bribe officials to have months lengthened or shortened, so we have made progress.

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