tz database

The tz database is a collaborative compilation of information about the world's time zones, primarily intended for use with computer programs and operating systems. Paul Eggert is its current editor and maintainer, with the organizational backing of ICANN. The tz database is also known as tzdata, the zoneinfo database or IANA time zone database, and occasionally as the Olson database, referring to the founding contributor, Arthur David Olson. Its uniform naming convention for time zones, such as America/New_York and Europe/Paris, was designed by Paul Eggert. The database attempts to record historical time zones and all civil changes since 1970, the Unix time epoch. It also includes transitions such as daylight saving time, and also records leap seconds. The database, as well as some reference source code, is in the public domain. New editions of the database and code are published as changes warrant, usually several times per year. The tz database is published as a set of text files which list the rules and zone transitions in a human-readable format. For use, these text files are compiled into a set of platform-independent binary files—one per time zone. The reference source code includes such a compiler called zic (zone information compiler), as well as code to read those files and use them in standard application programming interfaces such as localtime() and mktime(). Within the tz database, a time zone is any national region where local clocks have all agreed since 1970. This definition concerns itself first with geographic areas which have had consistent local clocks. This is different from other definitions which concern themselves with consistent offsets from a prime meridian. Therefore, each of the time zones defined by the tz database may document multiple offsets from UTC, typically including both standard time and daylight saving time. In the time zone text files, each time zone has one or more 'zone lines' in one of the time zone text files. The first zone line for a time zone gives the name of the time zone; any subsequent zone lines for that time zone leave the name blank, indicating that they apply to the same zone as the previous line. Each zone line for a zone specifies, for a range of date and time, the offset to UTC for standard time, the name of the set of rules that govern daylight saving time (or a hyphen if standard time always applies), the format for time zone abbreviations, and, for all but the last zone line, the date and time at which the range of date and time governed by that line ends. The rules for daylight saving time are specified in named rule sets. Each rule set has one or more rule lines in the time zone text files. A rule line contains the name of the rule set to which it belongs, the first year in which the rule applies, the last year in which the rule applies (or 'only' if it applies only in one year or 'max' if it is the rule currently in effect), the type of year to which the rule applies ('-' if it applies to all years in the specified range, which is almost always the case, otherwise a name used as an argument to a script that indicates whether the year is of the specified type), the month in which the rule takes effect, the day on which the rule takes effect (which could either be a specific day or a specification such as 'the last Sunday of the month'), the time of day at which the rule takes effect, the amount of time to add to the offset to UTC when the rule is in effect, and the letter or letters to use in the time zone abbreviation (for example, 'S' if the rule governs standard time and 'D' if it governs daylight saving time). The time zones have unique names in the form 'Area/Location', e.g. 'America/New_York'. A choice was also made to use English names or equivalents, and to omit punctuation and common suffixes. The underscore character is used in place of spaces. Hyphens are used where they appear in the name of a location. The Area and Location names have a maximum length of 14 characters.