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zoneinfo – time zone information files




The directory /etc/zoneinfo contains time zone information files used by the timezone command (see timezone ). They are in standard Unix time zone file format as described below.

The time zone information files begin with bytes reserved for future use, followed by six four-byte signed values, written in a "standard" byte order (the high-order byte of the value is written first). These values are, in order:

The number of GMT/local indicators stored in the file.

The number of standard/wall indicators stored in the file.

The number of leap seconds for which data is stored in the file.

The number of "transition times" for which data is stored in the file.

The number of "local time types" for which data is stored in the file (must not be zero).

The number of characters of "time zone abbreviation strings" stored in the file.

The above header is followed by tzh_timecnt four-byte signed values, sorted in ascending order. These values are written in "standard" byte order. Each is used as a transition time at which the rules for computing local time change. Next come tzh_timecnt one-byte unsigned values; each one tells which of the different types of "local time" types described in the file is associated with the same-indexed transition time. These values serve as indices into an array of structures that appears next in the file; these structures are written as a four-byte signed tt_gmtoff member in a standard byte order, followed by a one-byte signed tt_isdst member and a one-byte unsigned tt_abbrind member. In each structure, tt_gmtoff gives the number of seconds to be added to GMT, tt_isdst tells whether this time is during a Daylight Savings Time period and tt_abbrind serves as an index into the array of time zone abbreviation characters that follow the structure(s) in the file.

Then there are tzh_leapcnt pairs of four-byte values, written in standard byte order; the first value of each pair gives the time at which a leap second occurs; the second gives the total number of leap seconds to be applied after the given time. The pairs of values are sorted in ascending order by time.

Then there are tzh_ttisstdcnt standard/wall indicators, each stored as a one-byte value; they tell whether the transition times associated with local time types were specified as standard time or wall clock time. A local time transition specified in standard time ignores any offset due to Daylight Savings Time. On the other hand, a time specified in wall clock time takes the prevailing value of Daylight Savings Time in to account.

Finally there are tzh_ttisgmtcnt GMT/local indicators, each stored as a one-byte value; they tell whether the transition times associated with local time types were specified as GMT or local time.



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