ttinfo

ttinfo is a data type used by the IANA time zone database (tzdb) and the associated tzcode library to describe a single local time rule within a time zone. It encodes the information that applies after a transition: the offset from Coordinated Universal Time (UTC), whether daylight saving time is in effect, and the textual abbreviation that represents the rule (such as EST or EDT). In many tzfile-based implementations, ttinfo entries are part of a compiled time zone file and are paired with a separate list of abbreviation strings.

In the tzfile format, the time zone data are organized into arrays of transitions and corresponding ttinfo entries. Each ttinfo entry typically conveys three bits of information: the offset, a flag indicating whether the rule is daylight saving time, and an index into the array of abbreviation strings. The abbreviation string provides a human-readable label for the rule (for example, "EST", "EDT", "PST", etc.). The offset is the number of seconds east of UTC, so negative values denote time zones west of UTC.

The runtime logic uses ttinfo in combination with the transition times: when a given timestamp falls under a particular transition, the associated ttinfo is consulted to compute local time by adding the offset to UTC and applying DST semantics as indicated, while also displaying the appropriate abbreviation. This structure enables efficient conversion between UTC and local time across historical changes in a zone’s rules.

ttinfo is specific to the tzdb/tzcode ecosystem and is a foundational element of how most Unix-like systems represent and apply time zone rules. Its layout can vary slightly between tzcode versions, but its role remains to describe a concrete local-time rule within a zone.