daylight saving time : Java Glossary

By putting forward the hands of the clock you shall not advance the hour.
~ Victor Hugo (born: 1802-02-26 died: 1885-05-22 at age: 83)

Sometimes called DST, summer time or war time. The British call it BST (British Summer Time), BST. The French call it heure d'été. The Germans call it Sommerzeit. The Dutch call it Zomertijd.

It is a scheme to get people to wake up earlier and go to bed earlier to save electricity. You set clocks ahead a hour (or more) in spring and set them back in the fall. Commonly, you set the clock an hour forward at 2:00 A.M. on the second Sunday in March), and set it back one hour at 2:00 A.M. on the first Sunday in November. You can remember with the mnemonic spring forward, fall back.

DST globally is not as simple as in North America. The rules vary over time and by jurisdiction.

In British Columbia, Canada, where I live, local time is UTC (Coordinated Universal Time/Temps Universel Coordonné) minus 8 hours for PST (Pacific Standard Time) and UTC minus 7 hours for PDT (Pacific Daylight Time). DST starts on the second Sunday in March and ends the second Sunday in November.

Newfoundland is odd in that it in not an even hour difference from GMT/UTC. Newfoundland Standard Time is UTC minus 3.5 hours. It also odd in that they do the DST flip at 00:01 AM rather that 02:00 like the rest of Canada.

Headaches

DST causes all kinds of headaches for computer programmers:

1. In the spring, there is a missing hour. If you are not careful your payroll program will pay people for an hour they did not work.
2. In the fall, the are two different times, both 1:30 AM. The first is 1:30 AM daylight time and the second, a hour later, 1:30 AM standard time. Any time keeping must maintain the distinction. The distinction only matters during one hour each year.
3. The dates that daylight saving starts and end change year by year, and city by city. Your only hope is to convert all times to GMT (Greenwich Mean Time), then do your elapsed time calculations, and use locale tables to convert them back for display to local time. The Java Timezone class contains the locale-specific information to do these calculations.
4. Microsoft did not correct W95/W98/Me/NT/W2K to account for the extended daylight saving introduced in 2007 in the USA. You can manually fix the problem by downloading TZEdit.exe then use it to configure the start and end dates on the relevant time zones to start three weeks earlier (2:00 A.M. on the second Sunday in March), and end one week later (2:00 A.M. on the first Sunday in November). Canada follows the USA.

Arguments for dropping DST

• Despite the assurances of the Bush administration, DST does not save energy. With the extra evening daylight hours, people drive to the mall or ballpark. It is a retail stimulus and a way to increase gas consumption.
• The original story was we did it for the farmers. However, they strenuously opposed it when it was introduced. It gave them one less hour after daybreak before the city markets opened.
• I wonder how many people have been killed by the anomalies when the time changes. I could imagine some incompetent maker of medical pumps giving a double dose, or skipping a dose.
• The complication of the DST shifts must be handled by every computer program that handles time 24/7. Programmers often do it correctly. The shifts complicate the code, increases the likelihood of bugs, increases costs and increases the likelihood of data errors.
• Since, in general, clocks do not automatically adjust for DST, the manual adjustments trickle in over the next few days, with some clocks not corrected at all. This creates a limbo period where time cannot be accurately determined, since one never knows for sure if a clock has been corrected already, or has had a double correction. Even official time sources can be off. Shaw TV for example set their clock ahead around midnight instead of 2:00 AM. radio-synched clocks are now relatively inexpensive. Perhaps they will eventually replace conventional clocks and DST shifts can be automatic and bang on punctual.
• Anyone who has to coordinate actions of many people should be tracking time in UTC.
• It is a full-time job just to track the changes in DST rules in various parts of the world. The information must be include in every international computer program. It is perpetually out-of-date.
• Putin commanded Russians to stop fiddling with their clocks. Oddly he did this in the summer. Russians are now stuck year round in DST.

Arguments for Keeping DST

• Abolishing it would require difficult-to-obtain political consensus.
• Debate would take away from much more important issues.
• It would require changing computer programs to make them simpler.
• DST shifts are a good time to resync manual clocks.
• DST shifts are a good time to replace batteries in smoke alarms.
• DST provides one more reason to store all dates in UTC format in databases.

Adjustment

Linux automatically probes time servers with NTP (Network Time Protocol) to keeps its clock in sync.

Someday, computers will have built-in GPS (Global Positioning System) units and they will be able to configure the local time zone automatically. But for now, you must do that manually in the control panel.

Gotchas

• On your first reboot after a daylight saving change, Vista may set your clock ahead two hours instead of one. You can fix the problem with SetClock which will also let you verify your time zone.
• In modern operating systems including NT/W2K/XP/W2003/Vista/W7-32/W7-64, if you manually change the time after a daylight saving change, instead of configuring the time zone and letting it happen automatically, you might confuse the OS (Operating System), with the result that file dates on all your files change by one hour.
• If you let Vista or Linux change the time by itself, your file dates should not appear to change at a DST boundary. This is because the timestamps are kept internally in UTC and files that were last updated in winter are always displayed in winter time (even in summer), and files last updated in summer are always displayed in summer time, even in winter.
• WinZip timestamps are not kept is UTC, but in local time. This means that DST changes will trigger WinZip archived files to be updated, since their local timestamps change, even when the absolute UTC timestamp does not.
• The rules for DST start and end change over time and different for different jurisdictions. The tables sometimes also have errors that are gradually corrected. Keep your Java JDK (Java Development Kit)s and JRE (Java Runtime Environment)s up to date by periodically downloading and applying the Sun DST Timezone updater.

Rant

However, other people hated the order, and deliberately screwed it up by inventing DST, with every jurisdiction deciding for itself on just how much DST to have. We are working our way back slowly but surely to the old system, that does not even have the uniformity of solar synching.

The Internet is shrinking the world further. To get ahead of the curve, perhaps we should set our watches to UTC, or at least the alternate time function, such as I have on my radio-synched desk clock.

You’d think, at least in airports, clocks would also display UTC as would flight schedules. Once people got used to this, they would not be confused by changing time zones as they moved west or east between zones. The lengths of flights would be clearer. Tracking tummy time would be easier.

It would be a Good Thing™ at least if there were a U.N. department of time that maintained the official list of time zones and their DST rules. It is a difficult job. It should not be duplicated.

To get an idea what a nightmare DST has created have a look at Sun’s table of DST changes. The list is so huge, it can never be accurate.

To make sense of time every computer program needs a giant database that not only tracks what every micro jurisdiction is doing to meddle with time, but all they have ever done. This is ridiculous.

Canadian DST

In most of Canada, DST begins and ends at 2 AM on the same days as the USA. However, in Newfoundland and Labrador, DST begins one minute after midnight local time on the second Sunday in March. On the first Sunday in November time returns to standard at one minute after midnight local time.

A few areas of Canada don’t use DST at all including Fort St. John, Charlie Lake, Taylor, Dawson Creek and Creston in British Columbia, and most of Saskatchewan (except Denare Beach and Creighton).

Other countries use different dates or don’t use DST at all. They may us the same time zone, but have different DST rules. That why you don’t configure your time zone directly in the OS, but rather chose a city with representative DST rules.

The TimeZone Database

Arthur David Olson dedicated his life to maintaining a database about timezones and DST. Individual cities change their minds annually about the begin and end date/times for DST. The boundaries of the world’s timezones are constantly being tweaked. Every international computer program in the world has to incorporate all this historical lore to function properly. Paul Eggert is the new editor and maintainer of the tz database. Oracle updates Java with the new information with each release of the JDK/JRE. If you use an old JDK/JRE, you will get out-of-date timezone/DST information.