Editing Daylight saving time (section)

===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>