libc/stdtime/tzfile.5

.\" $FreeBSD: head/contrib/tzcode/stdtime/tzfile.5 200832 2009-12-22 11:17:10Z edwin $
.Dd December 1, 2013
.Dt TZFILE 5
.Os
.Sh NAME
.Nm tzfile
.Nd timezone information
.Sh SYNOPSIS
.In tzfile.h
.Sh DESCRIPTION
The time zone information files used by
.Xr tzset 3
begin with the magic characters
.Dq Li TZif
to identify them as
time zone information files,
followed by a character identifying the version of the file's format
(as of 2013, either an ASCII
.Dv NUL
or a
.Sq Li 2 ,
or
.Sq Li 3 )
followed by fifteen bytes containing zeroes reserved for future use,
followed by six four-byte integer values
written in a
.Dq standard
byte order
(the high-order byte of the value is written first).
These values are,
in order:
.Pp
.Bl -tag -compact -width tzh_ttisstdcnt
.It Va tzh_ttisgmtcnt
The number of UT/local indicators stored in the file.
.It Va tzh_ttisstdcnt
The number of standard/wall indicators stored in the file.
.It Va tzh_leapcnt
The number of leap seconds for which data is stored in the file.
.It Va tzh_timecnt
The number of
.Dq transition times
for which data is stored
in the file.
.It Va tzh_typecnt
The number of
.Dq local time types
for which data is stored
in the file (must not be zero).
.It Va tzh_charcnt
The number of characters of
.Dq time zone abbreviation strings
stored in the file.
.El
.Pp
The above header is followed by
.Va tzh_timecnt
four-byte signed integer values sorted in ascending order.
These values are written in
.Dq standard
byte order.
Each is used as a transition time (as returned by
.Xr time 3 )
at which the rules for computing local time change.
Next come
.Va tzh_timecnt
one-byte unsigned integer values;
each one tells which of the different types of
.Dq local time
types
described in the file is associated with the same-indexed transition time.
These values serve as indices into an array of
.Fa ttinfo
structures (with
.Fa tzh_typecnt
entries) that appears next in the file;
these structures are defined as follows:
.Bd -literal -offset indent
struct ttinfo {
	int32_t		tt_gmtoff;
	unsigned char	tt_isdst;
	unsigned char	tt_abbrind;
};
.Ed
.Pp
Each structure is written as a four-byte signed integer value for
.Fa tt_gmtoff ,
in a standard byte order, followed by a one-byte value for
.Va tt_isdst
and a one-byte value for
.Va tt_abbrind .
In each structure,
.Va tt_gmtoff
gives the number of seconds to be added to UT,
.Li tt_isdst
tells whether
.Li tm_isdst
should be set by
.Xr localtime 3
and
.Va tt_abbrind
serves as an index into the array of time zone abbreviation characters
that follow the
.Li ttinfo
structure(s) in the file.
.Pp
Then there are
.Va tzh_leapcnt
pairs of four-byte values, written in standard byte order;
the first value of each pair gives the time
(as returned by
.Xr time 3 )
at which a leap second occurs;
the second gives the
.Em total
number of leap seconds to be applied after the given time.
The pairs of values are sorted in ascending order by time.
.Pp
Then there are
.Va 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,
and are used when a time zone file is used in handling POSIX-style
time zone environment variables.
.Pp
Finally there are
.Va tzh_ttisgmtcnt
UT/local indicators, each stored as a one-byte value;
they tell whether the transition times associated with local time types
were specified as UT or local time,
and are used when a time zone file is used in handling POSIX-style
time zone environment variables.
.Pp
.Xr localtime 3
uses the first standard-time
.Li ttinfo
structure in the file
(or simply the first
.Li ttinfo
structure in the absence of a standard-time structure)
if either
.Li tzh_timecnt
is zero or the time argument is less than the first transition time recorded
in the file.
.Pp
For version-2-format time zone files,
the above header and data are followed by a second header and data,
identical in format except that eight bytes are used for each
transition time or leap second time.
After the second header and data comes a newline-enclosed,
POSIX-TZ-environment-variable-style string for use in handling instants
after the last transition time stored in the file
(with nothing between the newlines if there is no POSIX representation for
such instants).
.Pp
For version-3-format time zone files, the POSIX-TZ-style string may
use two minor extensions to the POSIX TZ format, as described in
.Xr tzset 3 .
First, the hours part of its transition times may be signed and range from
-167 through 167 instead of the POSIX-required unsigned values
from 0 through 24.
Second, DST is in effect all year if it starts
January 1 at 00:00 and ends December 31 at 24:00 plus the difference
between daylight saving and standard time.
.Pp
Future changes to the format may append more data.
.Sh SEE ALSO
.Xr ctime 3 ,
.Xr time2posix 3 ,
.Xr tzset 3 ,
.Xr zdump 8 ,
.Xr zic 8
.\" This file is in the public domain, so clarified as of
.\" 1996-06-05 by Arthur David Olson.