Editing Daylight saving time (section)

==Computing==
[[File:Daylightsavings.svg|thumb|upright|
alt=Strong man in sandals and with shaggy hair, facing away from audience/artist, grabbing a hand of a clock bigger than he is and attempting to force it backwards. The clock uses Roman numerals and the man is dressed in stripped-down Roman gladiator style. The text says "You can't stop time... But you can turn it back one hour at 2 a.m. on Oct. 28 when daylight-saving time ends and standard time begins."|
A 2001 US [[Public service advertising|public service advertisement]] reminded people to adjust clocks.]]

Changes to DST rules cause problems in existing computer installations. For example, the [[Energy Policy Act of 2005#Change to daylight saving time|2007 change to DST rules]] in North America required that many computer systems be upgraded, with the greatest onus on e-mail and calendar programs. The upgrades required a significant effort by corporate [[Information technology|information technologists]].<ref>{{cite news |author=Steve Lohr |title=Time change a 'mini-Y2K' in tech terms |url=https://www.nytimes.com/2007/03/05/technology/05daylight.html |work=[[The New York Times]] |date=5 March 2007 |access-date=21 February 2017 |archive-date=1 February 2017 |archive-url=https://web.archive.org/web/20170201213854/http://www.nytimes.com/2007/03/05/technology/05daylight.html |url-status=live }}</ref>

Some applications standardize on UTC to avoid problems with clock shifts and time zone differences.<ref>{{cite conference |author1=A. Gut |author2=L. Miclea |author3=Sz. Enyedi |author4=M. Abrudean |author5=I. Hoka |title=Database globalization in enterprise applications |book-title=2006 IEEE International Conference on Automation, Quality and Testing, Robotics |year=2006 |pages=356–359 }}</ref> Likewise, most modern operating systems internally handle and store all times as UTC and only convert to local time for display.<ref>{{cite web |author=Ron Bean |url=http://tldp.org/HOWTO/Clock-2.html |title=The Clock Mini-HOWTO |date=November 2000 |access-date=10 January 2012 |archive-date=13 January 2012 |archive-url=https://web.archive.org/web/20120113081243/http://tldp.org/HOWTO/Clock-2.html |url-status=live }}</ref><ref>{{cite web |author=Raymond Chen |url=https://devblogs.microsoft.com/oldnewthing/20040902-00/?p=37983 |title=Why does Windows keep your BIOS clock on local time? |date=November 2000 |access-date=10 January 2012 |archive-date=3 January 2012 |archive-url=https://web.archive.org/web/20120103015240/http://blogs.msdn.com/b/oldnewthing/archive/2004/09/02/224672.aspx |url-status=live }}</ref> However, even if UTC is used internally, the systems still require external [[leap second]] updates and time zone information to correctly calculate local time as needed. Many systems in use today base their date/time calculations from data derived from the [[tz database]] also known as zoneinfo.

===IANA time zone database===
The [[Internet Assigned Numbers Authority]] (IANA) [[tz&nbsp;database]] maps a name to the named location's historical and predicted clock shifts. This database is used by many computer software systems, including most [[Unix-like]] operating systems, [[Java (Sun)|Java]], and the [[Oracle RDBMS]];<ref>{{cite web |author1=Paul Eggert |author2=Arthur David Olson |url=http://www.twinsun.com/tz/tz-link.htm |title=Sources for time zone and daylight saving time data |date=30 June 2008 |url-status=dead |archive-url=https://web.archive.org/web/20120623013511/http://www.twinsun.com/tz/tz-link.htm |archive-date=23 June 2012 }}</ref> [[Hewlett-Packard|HP]]'s "tztab" database is similar but incompatible.<ref>{{cite book |publisher=Hewlett–Packard Co |title=HP-UX Reference: HP-UX 11i Version 3 |chapter-url=http://h20000.www2.hp.com/bc/docs/support/SupportManual/c02271674/c02271674.pdf |chapter=tztab(4) |year=2010 |url-status=dead |archive-url=https://web.archive.org/web/20130921054222/http://h20000.www2.hp.com/bc/docs/support/SupportManual/c02271674/c02271674.pdf |archive-date=21 September 2013 }}</ref> When temporal authorities change DST rules, zoneinfo updates are installed as part of ordinary system maintenance. In Unix-like systems the TZ [[environment variable]] specifies the [[List of tz zones by name|location name]], as in <code>TZ=':America/New_York'</code>. In many of those systems there is also a system-wide setting that is applied if the TZ environment variable is not set: this setting is controlled by the contents of the <code>/etc/localtime</code> file, which is usually a [[symbolic link]] or [[hard link]] to one of the zoneinfo files. Internal time is stored in time-zone-independent [[Unix time]]; the TZ is used by each of potentially many simultaneous users and processes to independently localize time display.

Older or stripped-down systems may support only the TZ values required by [[POSIX]], which specify at most one start and end rule explicitly in the value. For example, <code>TZ='EST5EDT,M3.2.0/02:00,M11.1.0/02:00'</code> specifies time for the eastern United States starting in 2007. Such a TZ value must be changed whenever DST rules change, and the new value applies to all years, mishandling some older timestamps.<ref>{{cite book |url=http://www.unix.org/version3/ |publisher=The Open Group |title=IEEE Std 1003.1–2004 |year=2004 |chapter-url=http://www.opengroup.org/susv3/basedefs/xbd_chap08.html#tag_08_03 |chapter=Other environment variables |access-date=17 February 2007 |archive-date=6 July 2010 |archive-url=https://web.archive.org/web/20100706063716/http://www.unix.org/version3/ |url-status=live }}</ref>