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)

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 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

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, 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.

Adjustment

Linux automatically probes time servers with NTP to keeps its clock in sync.

Someday, computers will have built-in GPS 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/W2K3/Vista/W7, if you manually change the time after a daylight saving change, instead of configuring the timezone and letting it happen automatically, you you might confuse the OS, 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 summmer), 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 and different for different jurisdictions. The tables sometimes also have errors that are gradually corrected. Keep your Java JDKs and JREs up to date by periodically downloading and applying the Sun DST Timezone updater.

Rant

However, other people hate 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 timezones 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.