Computing Changes to DST rules cause problems in existing computer installations. For example, the 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 technologists. Some applications standardize on UTC to avoid problems with clock shifts and time zone differences. Likewise, most modern operating systems internally handle and store all times as UTC and only convert to local time for display. 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 tz 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, and the Oracle RDBMS; HP's "tztab" database is similar but incompatible. 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 location name, as in TZ=':America/New_York'. 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 /etc/localtime 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, TZ='EST5EDT,M3.2.0/02:00,M11.1.0/02:00' 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. Permanent daylight saving time A move to permanent daylight saving time (staying on summer hours all year with no time shifts) is sometimes advocated and is currently implemented in some jurisdictions such as Argentina, Belarus, Iceland, Kyrgyzstan, Morocco, Namibia, Saskatchewan, Singapore, Turkey, Turkmenistan, Uzbekistan and Yukon. Although Saskatchewan follows Central Standard Time, its capital city Regina experiences solar noon