Daylight Saving Time : Java Glossary


daylight saving time

Daylight Saving Ends

Last night, Sunday 2016-11-06 at 2AM, you should have moved your wall clocks, alarm clocks, watches, timers and appliance clocks back one hour to 1AM. In the strange world of DST, the clock sweeps between 1 and 2AM twice.

  • Your computer should change your clock automatically. If it fails to show the correct time, use the SetClock to correct it from an atomic clock on the web. If you do it manually, chances are you will leap too many hours.
  • If you have Windows, read the Windows Time Entry to learn how to configure your clock to stay in perfect sync for the life of your computer.
  • If you own a radio-synced wall clock or watch it should automatically correct itself.
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 (Daylight Saving Time), 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.


In North America, in the spring you set the wall clocks ahead and in the fall you set them back. Think “Spring forward, fall back“. In the fall at 2AM on a designated Sunday, you roll the clock back to 1AM, giving you a duplicate hour between 1 and 2AM to sleep. Think about bears hibernating in the fall and getting extra sleep. In the spring at 2AM on a designated Sunday, you roll the wall clocks forward to 3AM, skipping out the entire hour between 2 and 3 AM. So you get an hour’s less sleep. You get up an hour earlier, measured by the sun. Think of yourself springing eagerly from bed an hour earlier than usual.

DST Around the World

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 AM like the rest of Canada.


DST causes all kinds of headaches for computer programmers:

Arguments for dropping DST

Arguments for Keeping DST



On daylight saving switch days, don’t set your computer clock forward an hour! Instead, make sure your machine is configured to the correct time zone and reset your clock from an atomic time source. It will automatically jump at 2 AM. If you fiddle the clock, you will screw up the file timestamps which are kept in time zone-independent, DST-independent, GMT aka UTC. To make sure your time zone is configured correctly and to set your clock from an atomic timesource, use the SetClock utility. Please see the Windows Time entry on how to configure your Windows clock and have it keep accurate time.

Windows Vista, W2008, W7-32, W7-64, W8-32, W8-64, W2012, W10-32 and W10-64 is supposed to automatically keep your clock in sync with a Microsoft time server, but for me it does not work. I suspect the problem is it is overloaded. I got it to work by configuring a non-Microsoft pool timeserver.

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.


  • 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, W2008, W7-32, W7-64, W8-32, W8-64, W2012, W10-32 and W10-64, 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 JDKs (Java Development Kits) and JREs (Java Runtime Environments) up to date by periodically downloading and applying the Sun DST Timezone updater.


Prior to standard time, every town kept its own local time. You could imagine the difficulty that caused once the telegraph and trains were shrinking the world. So Sir Sandford Fleming invented standard time zones. The boundaries have jogs to suit the political realities but it basically brought as much order to the chaos as you could reasonably expect.

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-synced 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 Oracle’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).

Daylight Savings Change Dates in the USA and Canada
year DST starts DST ends
2008 March 9 November 2
2009 March 8 November 1
2010 March 14 November 7
2011 March 13 November 6
2012 March 11 November 4
2013 March 10 November 3
2014 March 9 November 2
2015 March 8 November 1
2016 March 13 November 6
2017 March 12 November 5
2018 March 11 November 4
2019 March 10 November 3
2020 March 8 November 1

Other countries use different dates or don’t use DST at all. They may use the same time zone, but have different DST rules. That’s 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 (Java Development Kit)/JRE (Java Runtime Environment). If you use an old JDK/ JRE, you will get out-of-date timezone/DST information. He had some legal trouble with an astrology company that claimed ownership of such data. Happily, Oslon prevailed in the legal challenge. New versions of Java incorporate the latest timezone trivia information. Oracle also releases timezone updates for older versions.

atomic clock
Configure Windows Date format
DST dates start/stop: by country
Holiday Calculator: calculates DST start/end for any year
National Research Council of Canada Calendar/Time FAQ: complete, well written and illustrated
Negative health effects of DST time changes
radio-synced clock
Sun DST Timezone updater: update all your JDKs/JREs to latest DST/time zone tables
time sources
Windows Time: configuring and setting your clock in Windows

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