TZif

TZif, short for Time Zone Information Format, is a binary file format used to store time zone data for the IANA Time Zone Database (tzdata). It is the mechanism by which Unix-like systems determine local time, daylight saving transitions, and related rules for any given region. TZif files are typically located in the zoneinfo directory, such as /usr/share/zoneinfo, and are read by standard time libraries to convert between UTC and local times.

A TZif file begins with the ASCII signature “TZif” and a version character, which identifies the layout and capabilities of the data (versions include 1 and later 2, 3, 4). The header is followed by a series of data blocks that describe how local time is obtained. The blocks include transition times (epoch seconds indicating when a change occurs), an array of transition types that point to a list of time types, and a table of ttinfo records that specify each type’s UTC offset, whether it is daylight saving time, and an index into the abbreviation strings for display. The zone abbreviation strings themselves are stored in a separate block. In newer TZif formats, 64-bit times are used to extend the representable range, and leap second data may be included in separate sections.

The practical result is that time library code can apply the most recent transition before a given instant and use the corresponding type to compute the correct local time. TZif provides a compact, machine-readable representation of the rules governing time zones worldwide and is the de facto standard used by many operating systems.