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@database tzc.guide
@author Carsten Larsen
@(c) Carsten Larsen
@$VER: tz.guide 5.00 (15.10.2016)
@node Main "Amiga time zone database"
@title "Amiga time zone database"
"What time is it?" -- Richard Deacon as The King
"Any time you want it to be." -- Frank Baxter as The Scientist
(from the Bell System film "About Time")
The Time Zone Database (often called tz or zoneinfo) contains code and data that
represent the history of local time for many representative locations around the
globe. It is updated periodically to reflect changes made by political bodies to
time zone boundaries, UTC offsets, and daylight-saving rules.
@{b}USER MANUALS@{ub}
@{"Install " link "Install"} Install Amiga time zone database
@{"TimeZone " link "TimeZone"} Select a time zone
@{"TimeZoneInfo " link "TimeZoneInfo"} Show active time zone
@{"SetClockGMT " link "SetClockGMT"} Set battery backed-up hardware clock in GMT
@{"DSTCheck " link "DSTCheck"} Warn when daylight saving time changes
@{"ZDump " link "ZDump"} Time zone dumper
@{b}DEVELOPER MANUALS@{ub}
@{"Library " link "library"} Time zone library
@{"Zic " link "zic"} Time zone compiler
@{"tzfile " link "tzfile"} Time zone information
@{b}PROJECT MANUALS@{ub}
@{"Contributing " link "CONTRIBUTING"} Contributing to the tz code and data
@{"License " link "LICENSE"} Authors and terms
@endnode
@node Install "Install Amiga time zone database"
@title "Install Amiga time zone database"
@{b}INSTALL@{ub}
Minimum
* Copy timezone.library to Libs:
* Assign ZONEINFO: to your zoneinfo directory
* Copy zoneinfo from tzdata.lha to ZONEINFO:
Optionally
* Copy SetClockGMT to your systems C:
* Copy TimeZone to your systems SYS:Prefs
* Assign ZONEINFO: to LOCALE:Zoneinfo in S:Startup-Sequence
* Add SetClockGMT <NIL: >NIL: LOAD to end of S:Startup-Sequence
@{b}SETUP@{ub}
Select a time zone in TimeZone GUI:
TimeZone
Save GMT time to RTC (hardware) clock:
SetClockGMT SAVE
Load GMT time to locale timezone:
Add SetClockGMT LOAD to end of S:Startup-Sequence
Validate time zone preferences in shell:
TimeZoneInfo
Show time in other time zones:
Execute script/Show
@endnode
@node LICENSE "License and credits"
@title "License and credits"
@{b}LICENSE AND CREDITS@{ub}
Unless otherwise specified, all files in the tz code and data are in the public
domain, so clarified as of 2009-05-17 by Arthur David Olson. The few exceptions
are code derived from BSD, which uses the BSD license.
Thanks to these Time Zone Caballeros who've made major contributions to the time
conversion package: Keith Bostic; Bob Devine; Paul Eggert; Robert Elz; Guy Harris;
Mark Horton; John Mackin; and Bradley White.
Thanks also to Michael Bloom, Art Neilson, Stephen Prince, John Sovereign, and
Frank Wales for testing work, and to Gwillim Law for checking local mean time
data. Thanks in particular to Arthur David Olson, the project's founder and first
maintainer, to whom the time zone community owes the greatest debt of all.
None of them are responsible for remaining errors.
@endnode
@node CONTRIBUTING "Contributing to the tz code and data"
@title "Contributing to the tz code and data"
@{bg filltext}@{b}Contributing Contributing@{ub}@{bg background}
@{b}Contributing to the tz code and data (Non-Amiga specific)@{ub}
The time zone database is by no means authoritative: governments change timekeeping
rules erratically and sometimes with little warning, the data entries do not cover
all of civil time before 1970, and undoubtedly errors remain in the code and data.
Feel free to fill gaps or fix mistakes, and please email improvements to
tz@iana.org for use in the future.
To email small changes, please run a POSIX shell command like
'diff -u old/europe new/europe >myfix.patch', and attach
myfix.patch to the email.
For more-elaborate changes, please read the Theory file and browse the mailing list
archives <http://mm.icann.org/pipermail/tz/> for examples of patches that tend to
work well. Ideally, additions to data should contain commentary citing reliable
sources as justification.
Please submit changes against either the latest release in
<ftp://ftp.iana.org/tz/> or the master branch of the experimental Git repository.
If you use Git the following workflow may be helpful:
* Copy the experimental repository.
git clone https://github.com/eggert/tz.git
cd tz
* Get current with the master branch.
git checkout master
git pull
* Switch to a new branch for the changes. Choose a different
branch name for each change set.
git checkout -b mybranch
* Edit source files. Include commentary that justifies the
changes by citing reliable sources.
* Debug the changes, e.g.:
make check
make install
./zdump -v America/Los_Angeles
* For each separable change, commit it in the new branch, e.g.:
git add northamerica
git commit
See recent 'git log' output for the commit-message style.
* Create patch files 0001-*, 0002-*, ...
git format-patch master
* After reviewing the patch files, send the patches to tz@iana.org
for others to review.
git send-email master
* Start anew by getting current with the master branch again
(the second step above).
Please do not create issues or pull requests on GitHub, as the proper procedure for
proposing and distributing patches is via email as illustrated above.
@endnode
@node TimeZone "TimeZone user manual"
@title "TimeZone user manual"
@{bg filltext}@{b}TimeZone TimeZone@{ub}@{bg background}
@{b}NAME@{ub}
TimeZone - select a time zone
@{b}USAGE@{ub}
TimeZone
@{b}DESCRIPTION@{ub}
TimeZone is used to set local time zone preferences. Time zone can be
selected from a set of predefined zone files. The selected zone need to
be located in the ZONEINFO: directory.
The time zone name will be save to the TZ environment variable if selected.
Do not choose this option if you already have a rule in your TZ variable.
@endnode
@node TimeZoneInfo "TimeZoneInfo user manual"
@title "TimeZoneInfo user manual"
@{bg filltext}@{b}TimeZoneInfo TimeZoneInfo@{ub}@{bg background}
@{b}NAME@{ub}
TimeZoneInfo - Show active time zone
@{b}USAGE@{ub}
TimeZoneInfo
@{b}DESCRIPTION@{ub}
TimeZoneInfo lists the most important properties of the active time
zone.
@endnode
@node SetClockGMT "SetClockGMT user manual"
@title "SetClockGMT user manual"
@{bg filltext}@{b}SetClockGMT SetClockGMT@{ub}@{bg background}
@{b}NAME@{ub}
SetClockGMT - Set or read the battery backed-up hardware clock in GMT
@{b}USAGE@{ub}
SetClockGMT LOAD|SAVE|RESET
@{b}DESCRIPTION@{ub}
SetClockGMT SAVE sets the date and time of the battery backed-up
hardware clock from the current system time and adjust it according
to locale time zone. In order for SetClockGMT to operate correct, a
local time zone need to be selected first.
SetClockGMT LOAD sets the current system time in locale time zone
from the battery backed-up clock. In systems using Kickstart 2.0 or
later, the time is set automatically during the boot process.
SetClockGMT LOAD need to executed after boot in order to active locale
time zone.
The RESET option resets the clock completely. This may be necessary if
a poorly written program that does not follow the rules turns the clock
off or sets the test bit of the clock.
@endnode
@node DSTCheck "DSTCheck user manual"
@title "DSTCheck user manual"
@{bg filltext}@{b}DSTCheck DSTCheck@{ub}@{bg background}
@{b}NAME@{ub}
DSTCheck - Warn when daylight saving time changes
@{b}USAGE@{ub}
DSTCheck
@{b}DESCRIPTION@{ub}
DSTCheck can be used in scripts to track when daylight saving time
changes.
@{b}EXAMPLE@{ub}
DSTCheck
IF WARN
echo "### Summertime ###"
ELSE
echo "### Wintertime ###"
ENDIF
@endnode
@node library "Timezone Library"
@title "Timezone Library"
@{b}timezone.library@{ub}
Version 5 of the Timezone Library implements the following functions.
time.h tzset, tzalloc, tzfree, time, mktime, mktime_z, localtime, localtime_r,
localtime_rz, gmtime, gmtime_r, ctime, ctime_r, ctime_rz, difftime,
offtime, offtime_r, timeoff, asctime, asctime_r, strftime, strftime_l,
strptime, strptime_l, timegm, timelocal, timelocal_z, daylight_c,
timezone_c, altzone_c, tzname_c, time2posix, time2posix_z, posix2time,
posix2time_z, tzsetwall, tzgetname
sys/time.h gettimeofday, settimeofday
extra getsystime, setsystime, tzgetlocation, gmtoffset, stou, utos
@endnode
@node ZDump "ZDump user manual"
@title "ZDump user manual"
@{bg filltext}@{b}ZDump ZDump@{ub}@{bg background}
@{b}NAME@{ub}
ZDump - time zone dumper
@{b}USAGE@{ub}
ZDump [VERBOSE|VERBOSE2] [C=CUTOFF] [T=CUTOFFVERBOSE] { zonenames }
@{b}DESCRIPTION@{ub}
ZDump prints the current time in each zonename named on the command
line.
@{b}OPTIONS@{ub}
VERBOSE
For each zonename on the command line, print the time at the
lowest possible time value, the time one day after the lowest
possible time value, the times both one second before and
exactly at each detected time discontinuity, the time at one day
less than the highest possible time value, and the time at the
highest possible time value. Each line is followed by isdst=D
where D is positive, zero, or negative depending on whether the
given time is daylight saving time, standard time, or an unknown
time type, respectively. Each line is also followed by gmtoff=N
if the given local time is known to be N seconds east of
Greenwich.
VERBOSE2
Like -v, except omit the times relative to the extreme time
values. This generates output that is easier to compare to that
of implementations with different time representations.
CUTOFF [loyear,]hiyear
Cut off verbose output at the given year(s). Cutoff times are
computed using the proleptic Gregorian calendar with year 0 and
with Universal Time (UT) ignoring leap seconds. The lower bound
is exclusive and the upper is inclusive; for example, a loyear
of 1970 excludes a transition occurring at 1970-01-01 00:00:00
UTC but a hiyear of 1970 includes the transition. The default
cutoff is -500,2500.
CUTOFFVERBOSE [lotime,]hitime
Cut off verbose output at the given time(s), given in decimal
seconds since 1970-01-01 00:00:00 Coordinated Universal Time
(UTC). The zonename determines whether the count includes leap
seconds. As with -c, the cutoff's lower bound is exclusive and
its upper bound is inclusive.
@{b}LIMITATIONS@{ub}
Time discontinuities are found by sampling the results returned by
localtime at twelve-hour intervals. This works in all real-world
cases; one can construct artificial time zones for which this fails.
In the output, "UT" denotes the value returned by gmtime, which uses
UTC for modern time stamps and some other UT flavor for time stamps
that predate the introduction of UTC. No attempt is currently made
to have the output use "UTC" for newer and "UT" for older time stamps,
partly because the exact date of the introduction of UTC is
problematic.
@{b}SEE ALSO@{ub}
@{"tzfile" link "tzfile"}, @{"zic" link "zic"}
@endnode
@node zic "zic developer manual"
@title "Time zone compiler"
@{bg filltext}@{b}ZIC ZIC@{ub}@{bg background}
@{b}NAME@{ub}
zic - time zone compiler
@{b}USAGE@{ub}
zic [ option ... ] [ filename ... ]
@{b}DESCRIPTION@{ub}
Zic reads text from the file(s) named on the command line and creates
the time conversion information files specified in this input. If a
filename is "-", the standard input is read.
@{b}OPTIONS@{ub}
These options are available:
--version
Output version information and exit.
-d directory
Create time conversion information files in the named directory
rather than in the standard directory named below.
-l timezone
Use the given time zone as local time. Zic will act as if the
input contained a link line of the form
Link timezone localtime
-p timezone
Use the given time zone's rules when handling POSIX-format time
zone environment variables. Zic will act as if the input
contained a link line of the form
Link timezone posixrules
-L leapsecondfilename
Read leap second information from the file with the given name.
If this option is not used, no leap second information appears
in output files.
-v Be more verbose, and complain about the following situations:
The input specifies a link to a link.
A year that appears in a data file is outside the range of years
representable by time(2) values.
A time of 24:00 or more appears in the input. Pre-1998 versions
of zic prohibit 24:00, and pre-2007 versions prohibit times
greater than 24:00.
A rule goes past the start or end of the month. Pre-2004
versions of zic prohibit this.
The output file does not contain all the information about the
long-term future of a zone, because the future cannot be
summarized as an extended POSIX TZ string. For example, as of
2013 this problem occurs for Iran's daylight-saving rules for
the predicted future, as these rules are based on the Iranian
calendar, which cannot be represented.
The output contains data that may not be handled properly by
client code designed for older zic output formats. These
compatibility issues affect only time stamps before 1970 or
after the start of 2038.
A time zone abbreviation has fewer than 3 characters. POSIX
requires at least 3.
An output file name contains a byte that is not an ASCII letter,
"-", "/", or "_"; or it contains a file name component that
contains more than 14 bytes or that starts with "-".
-s Limit time values stored in output files to values that are the
same whether they're taken to be signed or unsigned. You can
use this option to generate SVVS-compatible files.
-y command
Use the given command rather than yearistype when checking year
types (see below).
Input files should be text files, that is, they should be a series of
zero or more lines, each ending in a newline byte and containing at
most 511 bytes, and without any NUL bytes. The input text's encoding
is typically UTF-8 or ASCII; it should have a unibyte representation
for the POSIX Portable Character Set (PPCS) <http://pubs.opengroup.org/
onlinepubs/9699919799/basedefs/V1_chap06.html> and the encoding's non-
unibyte characters should consist entirely of non-PPCS bytes. Non-PPCS
characters typically occur only in comments: although output file names
and time zone abbreviations can contain nearly any character, other
software will work better if these are limited to the restricted syntax
described under the -v option.
Input lines are made up of fields. Fields are separated from one
another by one or more white space characters. The white space
characters are space, form feed, carriage return, newline, tab, and
vertical tab. Leading and trailing white space on input lines is
ignored. An unquoted sharp character (#) in the input introduces a
comment which extends to the end of the line the sharp character
appears on. White space characters and sharp characters may be
enclosed in double quotes (") if they're to be used as part of a field.
Any line that is blank (after comment stripping) is ignored. Non-blank
lines are expected to be of one of three types: rule lines, zone lines,
and link lines.
Names (such as month names) must be in English and are case
insensitive. Abbreviations, if used, must be unambiguous in context.
A rule line has the form
Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S
For example:
Rule US 1967 1973 - Apr lastSun 2:00 1:00 D
The fields that make up a rule line are:
NAME Gives the (arbitrary) name of the set of rules this rule is
part of.
FROM Gives the first year in which the rule applies. Any integer
year can be supplied; the proleptic Gregorian calendar is
assumed. The word minimum (or an abbreviation) means the
minimum year representable as an integer. The word maximum (or
an abbreviation) means the maximum year representable as an
integer. Rules can describe times that are not representable
as time values, with the unrepresentable times ignored; this
allows rules to be portable among hosts with differing time
value types.
TO Gives the final year in which the rule applies. In addition to
minimum and maximum (as above), the word only (or an
abbreviation) may be used to repeat the value of the FROM
field.
TYPE Gives the type of year in which the rule applies. If TYPE is
"-" then the rule applies in all years between FROM and TO
inclusive. If TYPE is something else, then zic executes the
command
yearistype year type
to check the type of a year: an exit status of zero is taken to
mean that the year is of the given type; an exit status of one
is taken to mean that the year is not of the given type.
IN Names the month in which the rule takes effect. Month names
may be abbreviated.
ON Gives the day on which the rule takes effect. Recognized forms
include:
5 the fifth of the month
lastSun the last Sunday in the month
lastMon the last Monday in the month
Sun>=8 first Sunday on or after the eighth
Sun<=25 last Sunday on or before the 25th
Names of days of the week may be abbreviated or spelled out in
full. Note that there must be no spaces within the ON field.
AT Gives the time of day at which the rule takes effect.
Recognized forms include:
2 time in hours
2:00 time in hours and minutes
15:00 24-hour format time (for times after noon)
1:28:14 time in hours, minutes, and seconds
- equivalent to 0
where hour 0 is midnight at the start of the day, and hour 24
is midnight at the end of the day. Any of these forms may be
followed by the letter w if the given time is local "wall
clock" time, s if the given time is local "standard" time, or u
(or g or z) if the given time is universal time; in the absence
of an indicator, wall clock time is assumed. The intent is
that a rule line describes the instants when a clock/calendar
set to the type of time specified in the AT field would show
the specified date and time of day.
SAVE Gives the amount of time to be added to local standard time
when the rule is in effect. This field has the same format as
the AT field (although, of course, the w and s suffixes are not
used).
LETTER/S
Gives the "variable part" (for example, the "S" or "D" in "EST"
or "EDT") of time zone abbreviations to be used when this rule
is in effect. If this field is "-", the variable part is null.
A zone line has the form
Zone NAME GMTOFF RULES/SAVE FORMAT [UNTILYEAR [MONTH [DAY [TIME]]]]
For example:
Zone Australia/Adelaide 9:30 Aus AC%sT 1971 Oct 31 2:00
The fields that make up a zone line are:
NAME The name of the time zone. This is the name used in creating the
time conversion information file for the zone. It should not
contain a file name component "." or ".."; a file name component
is a maximal substring that does not contain "/".
GMTOFF
The amount of time to add to UT to get standard time in this
zone. This field has the same format as the AT and SAVE fields
of rule lines; begin the field with a minus sign if time must be
subtracted from UT.
RULES/SAVE
The name of the rule(s) that apply in the time zone or,
alternately, an amount of time to add to local standard time. If
this field is - then standard time always applies in the time
zone.
FORMAT
The format for time zone abbreviations in this time zone. The
pair of characters %s is used to show where the "variable part"
of the time zone abbreviation goes. Alternately, a slash (/)
separates standard and daylight abbreviations.
UNTILYEAR [MONTH [DAY [TIME]]]
The time at which the UT offset or the rule(s) change for a
location. It is specified as a year, a month, a day, and a time
of day. If this is specified, the time zone information is
generated from the given UT offset and rule change until the time
specified, which is interpreted using the rules in effect just
before the transition. The month, day, and time of day have the
same format as the IN, ON, and AT fields of a rule; trailing
fields can be omitted, and default to the earliest possible value
for the missing fields.
The next line must be a "continuation" line; this has the same
form as a zone line except that the string "Zone" and the name
are omitted, as the continuation line will place information
starting at the time specified as the "until" information in the
previous line in the file used by the previous line.
Continuation lines may contain "until" information, just as zone
lines do, indicating that the next line is a further
continuation.
A link line has the form
Link TARGET LINK-NAME
For example:
Link Europe/Istanbul Asia/Istanbul
The TARGET field should appear as the NAME field in some zone line.
The LINK-NAME field is used as an alternate name for that zone; it has
the same syntax as a zone line's NAME field.
Except for continuation lines, lines may appear in any order in the
input. However, the behavior is unspecified if multiple zone or link
lines define the same name, or if the source of one link line is the
target of another.
Lines in the file that describes leap seconds have the following form:
Leap YEAR MONTH DAY HH:MM:SS CORR R/S
For example:
Leap 1974 Dec 31 23:59:60 + S
The YEAR, MONTH, DAY, and HH:MM:SS fields tell when the leap second
happened. The CORR field should be "+" if a second was added or "-" if
a second was skipped. The R/S field should be (an abbreviation of)
"Stationary" if the leap second time given by the other fields should
be interpreted as UTC or (an abbreviation of) "Rolling" if the leap
second time given by the other fields should be interpreted as local
wall clock time.
@{b}EXTENDED EXAMPLE@{ub}
Here is an extended example of zic input, intended to illustrate many
of its features.
# Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S
Rule Swiss 1941 1942 - May Mon>=1 1:00 1:00 S
Rule Swiss 1941 1942 - Oct Mon>=1 2:00 0 -
Rule EU 1977 1980 - Apr Sun>=1 1:00u 1:00 S
Rule EU 1977 only - Sep lastSun 1:00u 0 -
Rule EU 1978 only - Oct 1 1:00u 0 -
Rule EU 1979 1995 - Sep lastSun 1:00u 0 -
Rule EU 1981 max - Mar lastSun 1:00u 1:00 S
Rule EU 1996 max - Oct lastSun 1:00u 0 -
# Zone NAME GMTOFF RULES/SAVE FORMAT UNTIL
Zone Europe/Zurich 0:34:08 - LMT 1853 Jul 16
0:29:46 - BMT 1894 Jun
1:00 Swiss CE%sT 1981
1:00 EU CE%sT
Link Europe/Zurich Switzerland
In this example, the zone is named Europe/Zurich but it has an alias as
Switzerland. This example says that Zurich was 34 minutes and 8
seconds west of UT until 1853-07-16 at 00:00, when the legal offset was
changed to 7o26'22.50''; although this works out to 0:29:45.50, the
input format cannot represent fractional seconds so it is rounded here.
After 1894-06-01 at 00:00 Swiss daylight saving rules (defined with
lines beginning with "Rule Swiss") apply, and the UT offset became one
hour. From 1981 to the present, EU daylight saving rules have applied,
and the UTC offset has remained at one hour.
In 1941 and 1942, daylight saving time applied from the first Monday in
May at 01:00 to the first Monday in October at 02:00. The pre-1981 EU
daylight-saving rules have no effect here, but are included for
completeness. Since 1981, daylight saving has begun on the last Sunday
in March at 01:00 UTC. Until 1995 it ended the last Sunday in
September at 01:00 UTC, but this changed to the last Sunday in October
starting in 1996.
For purposes of display, "LMT" and "BMT" were initially used,
respectively. Since Swiss rules and later EU rules were applied, the
display name for the time zone has been CET for standard time and CEST
for daylight saving time.
@{b}NOTES@{ub}
For areas with more than two types of local time, you may need to use
local standard time in the AT field of the earliest transition time's
rule to ensure that the earliest transition time recorded in the
compiled file is correct.
If, for a particular zone, a clock advance caused by the start of
daylight saving coincides with and is equal to a clock retreat caused
by a change in UT offset, zic produces a single transition to daylight
saving at the new UT offset (without any change in wall clock time).
To get separate transitions use multiple zone continuation lines
specifying transition instants using universal time.
Time stamps well before the Big Bang are silently omitted from the
output. This works around bugs in software that mishandles large
negative time stamps. Call it sour grapes, but pre-Big-Bang time
stamps are physically suspect anyway. The pre-Big-Bang cutoff time
is approximate and may change in future versions.
zic is currently not included in the Amiga distribution.
@{b}FILE@{ub}
ZONEINFO: - standard directory used for created files
@{b}SEE ALSO@{ub}
@{"tzfile" link "tzfile"}, @{"zdump" link "zdump"}
@endnode
@node tzfile "tzfile developer manual"
@title "Time zone information"
@{bg filltext}@{b}TZFILE TZFILE@{ub}@{bg background}
@{b}NAME@{ub}
tzfile - time zone information
@{b}SYNOPSIS@{ub}
#include <tzfile.h>
@{b}DESCRIPTION@{ub}
The time zone information files used by tzset begin with the magic
characters "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 NUL, or '2', or '3') followed by fifteen
bytes containing zeroes reserved for future use, followed by six four-
byte integer values written in a standard byte order (the high-order
byte of the value is written first). These values are, in order:
tzh_ttisgmtcnt
The number of UT/local indicators stored in the file.
tzh_ttisstdcnt
The number of standard/wall indicators stored in the file.
tzh_leapcnt
The number of leap seconds for which data entries are stored in
the file.
tzh_timecnt
The number of transition times for which data entries are stored
in the file.
tzh_typecnt
The number of local time types for which data entries are stored
in the file (must not be zero).
tzh_charcnt
The number of characters of time zone abbreviation strings
stored in the file.
The above header is followed by tzh_timecnt four-byte signed integer
values sorted in ascending order. These values are written in standard
byte order. Each is used as a transition time (as returned by time) at
which the rules for computing local time change. Next come tzh_timecnt
one-byte unsigned integer 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 ttinfo structures (with tzh_typecnt entries) that appears next
in the file; these structures are defined as follows:
struct ttinfo {
int32_t tt_gmtoff;
unsigned char tt_isdst;
unsigned char tt_abbrind;
};
Each structure is written as a four-byte signed integer value for
tt_gmtoff, in a standard byte order, followed by a one-byte value for
tt_isdst and a one-byte value for tt_abbrind. In each structure,
tt_gmtoff gives the number of seconds to be added to UT, tt_isdst tells
whether tm_isdst should be set by localtime (3) and tt_abbrind serves
as an index into the array of time zone abbreviation characters that
follow the ttinfo 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 (as
returned by 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, and are used when a time zone file is used in handling POSIX-
style time zone environment variables.
Finally there are 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.
Localtime uses the first standard-time ttinfo structure in the file (or
simply the first ttinfo structure in the absence of a standard-time
structure) if either tzh_timecnt is zero or the time argument is less
than the first transition time recorded in the file.
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).
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
newtzset(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.
Future changes to the format may append more data.
@{b}SEE ALSO@{ub}
@{"zdump" link "zdump"}, @{"zic" link "zic"}
@endnode
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