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timezone(3)
NAME
daylight, timezone, tzname, tzset - sets and accesses time zone conversion
information
SYNOPSIS
#include <time.h>
void tzset(void);
extern int daylight;
extern long timezone;
extern char *tzname[];
LIBRARY
Standard C Library (libc.so, libc.a)
STANDARDS
Interfaces documented on this reference page conform to industry standards
as follows:
tzset(): POSIX.1, XSH4.2
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
DESCRIPTION
The tzset() function uses the value of the environment variable TZ to set
time conversion information used by several other functions, including
ctime(), ctime_r(), getdate(), getdate_r(), localtime(), localtime_r(),
mktime(), strftime(), and strptime().
If the TZ variable is not set, tzset() uses implementation-dependent
default time zone information. This information is located in the
/etc/zoneinfo/localtime file. See the section Time Zone Handling for
details.
The tzset() function sets the external variable tzname as follows:
tzname0 = "std";
tzname1 = "dst";
where std indicates the standard time zone and dst designates the
alternative time zone (such as Daylight Savings Time). (These variables are
described below in the section TZ Environment Variable.)
The tzset() function also sets the external variable daylight to 0 if
Daylight Savings Time conversions should never be applied for the time zone
in use. Otherwise, daylight is set to a nonzero value.
The external variable timezone is set to the difference, in seconds,
between Coordinated Universal Time (UTC) and local standard time. For
example:
______________
TZ timezone
______________
EST 5*60*60
GMT 0*60*60
JST -9*60*60
MET -1*60*60
MST 7*60*60
PST 8*60*60
______________
Time Zone Handling
The operating system uses a public-domain time zone handling package that
puts time zone conversion rules in easily accessible and modifiable files.
These files are in the directory /etc/zoneinfo/sources. The time zone
compiler zic(8) converts these files to a special format described in
tzfile(4) and places them in the /etc/zoneinfo directory. This format is
readable by the C library functions that handle time zone information.
The tzset() function uses the tzfile-formatted file linked by
/etc/zoneinfo/localtime to set the time zone conversion information. The
/etc/zoneinfo/localtime link is set during installation to a file in the
/etc/zoneinfo directory. For example, for time zone information consistent
with the city of New York on the American continent,
/etc/zoneinfo/localtime is linked to /etc/zoneinfo/America/New_York.
If the TZ environment variable is defined, the defined value overrides the
time zone information in /etc/zoneinfo/localtime. TZ can be set by a user
as a regular environment variable for converting to alternate time zones.
See the section TZ Environment Variable for details.
Getting Time Zone Information
The libc routines ctime() and localtime() return the local time and time
zone information. The ctime() routine returns a string that corresponds to
the local time; for example, Tue Oct 27 13:35:29 1992.
The localtime() routine returns a pointer to a tm structure (defined in
<sys/time.h>) that contains the local time expressed in fields of the tm
structure. For time zone information, there are three relevant fields:
tm_isdst
A option that is set to 1 if daylight savings time is currently in
effect. Otherwise, the option is set to 0.
tm_gmtoff
Seconds east of Greenwich. For example, -18000 means 5 hours west of
Greenwich.
tm_zone
Abbreviation for the current time zone (for example, EST, PDT, GMT).
Setting Time Zone Information
The /etc/zoneinfo/localtime link can be changed by the system administrator
to any file in the /etc/zoneinfo directory.
For example, the following command changes the local time zone to be
consistent with the city of New York on the American continent:
# ln -sf /etc/zoneinfo/America/New_York /etc/zoneinfo/localtime
Subsequent calls to the time zone related functions in libc (ctime() and
localtime()) use this link for the default time zone information.
If the time zone and daylight savings time information in the
/etc/zoneinfo/sources directory is incorrect for your time zone, you can
change the information in the source files and then use the zic command to
generate a corresponding /etc/zoneinfo file.
A user can override the default time zone information by setting the TZ
environment variable as described in the section TZ Environment Variable.
TZ Environment Variable
When TZ appears in the environment and its value is not a null string, the
value has one of three formats:
:
:pathname
stdoffset[dst[offset][,start[/time],end[/time]]]
[Tru64 UNIX] If TZ has the single colon format (:), Coordinated Universal
Time (UTC) is used.
[Tru64 UNIX] If TZ has the colon-pathname format (:pathname), the
characters following the colon specify the pathname of a tzfile(4) format
file from which to read the time conversion information. A pathname
beginning with a slash (/) represents an absolute pathname; otherwise, the
pathname is relative to the system time conversion information directory
/etc/zoneinfo.
If TZ does not begin with a colon (:), the components of the string are as
follows:
std and dst
Three or more characters that are the designation for the standard
(std) or alternative (dst) time zone (such as Daylight Savings Time).
Only std is required. If dst is not supplied, the alternative time does
not apply to the locale. Upper- and lowercase letters are explicitly
allowed. Any characters, except digits, a leading colon (:), comma (,),
minus (-), plus (+), and ASCII NUL, are allowed.
offset
Indicates the value to be added to the local time to arrive at GMT. The
offset has the form:
hh[:mm[:ss]]
The minutes (mm) and seconds (ss) are optional. The hour (hh) is
required and can be either one or two digits. The offset following std
is required. If no offset follows dst, the alternative time is assumed
to be one hour ahead of standard time. One or more digits can be used;
the value is always interpreted as a decimal number. The hour value
must be between zero and 24. The value for the minutes and seconds, if
present, must be between zero and 59. If preceded by a minus sign (-),
the time zone is east of the Prime Meridian; otherwise it is west,
which can be indicated by a preceding plus sign (+).
start and end
Indicates when to change to and return from alternative time. The
start argument is the date when the change from standard to alternative
time occurs; end is the date for changing back. If start and end are
not specified, the default is the US Daylight Saving Time start and end
dates. The format for start and end must be one of the following:
Jn The Julian day n (1 <= n <= 365). Leap days are not counted.
That is, in all years, including leap years, February 28 is day
59 and March 1 is day 60. It is impossible to explicitly refer
to February 29.
n The zero-based Julian day (0 <=n <= 365). Leap days are counted
making it possible to refer to February 29.
Mm.n.d The dth day (0 <= d <= 6) of week n of month m of the year (1
<= n <= 5, 1 <= m <= 12). When n is 5, it refers to the last d
day of month m which may occur in either the fourth or fifth
week. Week 1 is the first week in which the dth day occurs. Day
zero is Sunday.
time
Describes the time when, in current time, the change to or return from
alternative time occurs. The time parameter has the same format as
offset, except that there can be no leading minus (-) or plus (+) sign.
If time is not specified, the default is 02:00:00.
As an example, the TZ variable value EST5EDT4,M4.1.0,M10.5.0 describes the
rule defined in 1987 for the Eastern time zone in the US. EST (Eastern
Standard Time) is the designation for standard time, which is 5 hours
behind GMT. EDT (Eastern Daylight Time) is the designation for alternative
time, which is 4 hours behind GMT. EDT starts on the first Sunday in April
and ends on the last Sunday in October. In both cases, since time was not
specified, the changes occur at the default time, which is 2:00 A.M. Note
that the start and end dates did not need to be specified since they are
the defaults.
NOTES
[Tru64 UNIX] For users of the SVID2 habitat, TZ is defined by default in
the following format:
stdoffset[dst[offset]]
[Tru64 UNIX] For users of the SVID3 habitat, TZ is defined by default in
the following format:
:pathname
See the section TZ Environment Variable for definitions of the parameters
used in these formats.
SEE ALSO
Functions: ctime(3), ctime_r(3), difftime(3), getdate(3), getdate_r(3),
getenv(3), localtime(3), localtime_r(3), mktime(3), strftime(3),
strptime(3), time(3)
Commands: date(1), zdump(8), zic(8)
Files: tzfile(4)
Standards: standards(5)
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