Sets and accesses time-zone conversion.
Format
#include <time.h>
void tzset (void);
extern char *tzname[];
extern long int timezone;
extern int daylight;
1 – Description
The tzset function initializes time-conversion information
used by the ctime, localtime, mktime, strftime, and wcsftime
functions.
The tzset function sets the following external variables:
o tzname is set as follows, where "std" is a 3-byte name for
the standard time zone, and "dst" is a 3-byte name for the
Daylight Savings Time zone:
tzname[0] = "std"
tzname[1] = "dst"
o daylight is set to 0 if Daylight Savings Time should never be
applied to the time zone. Otherwise, daylight is set to 1.
o timezone is set to the difference between UTC and local
standard time.
The environment variable TZ specifies how tzset initializes time
conversion information:
o If TZ is absent from the environment, the implementation-
dependent time-zone information is used, as follows:
The best available approximation to local wall-clock time
is used, as defined by the SYS$LOCALTIME system logical,
which points to a tzfile format file that describes default
time-zone rules.
This system logical is set during the installation
of OpenVMS Version 7.0 or higher to define a
time-zone file based off the root directory
SYS$COMMON:[SYS$ZONEINFO.SYSTEM].
o If TZ appears in the environment but its value is a null
string, Coordinated Universal Time (UTC) is used (without
leap-second correction).
o If TZ appears in the environment and its value is not a null
string, the value has one of three formats, as described in
Time-Zone Initialization Rules.
Table REF-11 Time-Zone Initialization Rules
TZ Format Meaning
: UTC is used.
:pathname The characters following the colon specify the
pathname of a tzfile format file from which to
read the time-conversion information. A pathname
beginning with a slash (/) represents an absolute
pathname; otherwise, the pathname is relative to
the system time-conversion information directory
specified by SYS$TZDIR, which by default is
SYS$COMMON:[SYS$ZONEINFO.SYSTEM].
stdoffset[dstThefset]e is first used as the pathname of a file
(as described for the :pathname format) from which
[,rule]] to read the time-conversion information.
If that file cannot be read, the value is then
interpreted as a direct specification of the time-
conversion information, as follows:
std and dst-Three or more characters that are the
designation for the time zone:
o std-Standard time zone. Required.
o dst-Daylight Savings Time zone. Optional. If dst
is omitted, Daylight Savings Time does not apply.
Uppercase and lowercase letters are explicitly
allowed. Any characters are allowed, except the
following:
o digits
o leading colon (:)
o comma (,)
o minus (-)
o plus (+)
o ASCII null character
offset-The value added to the local time to arrive
at UTC. The offset has the following format:
hh[:mm[:ss]]
In this format:
o hh (hours) is a one-or two-digit value of 0-24.
o mm (minutes) is a value of 0-59. (optional)
o ss (seconds) is a value of 0-59. (optional)
The offset following std is required. If no offset
follows dst, summer time is assumed, one hour ahead
of standard time. You can use one or more digits;
the value is always interpreted as a decimal number.
If the time zone is preceded by a minus sign (-),
the time zone is East of Greenwich; otherwise, it
is West, which can also be indicated by a preceding
plus sign (+).
rule-Indicates when to change to and return from
summer time. The rule has the form:
start[/time], end[/time]
where:
o start is the date when the change from standard
time to summer time occurs.
o end is the date for returning from summer time to
standard time.
If start and end are omitted, the default is the
US Daylight Savings Time start and end dates.
The format for start and end must be one of the
following:
o 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. You cannot explicitly refer to February
29.
o n-The zero based Julian day (0 < n < 365). Leap
days are counted, making it possible to refer to
February 29.
o Mm.n.d-The nth d day of month m, where:
0 < n < 5
0 < d < 6
1 < m < 12
When n is 5, it refers to the last d day of month
m. Sunday is day 0.
time-The time when, in current time, the change to
or return from summer time occurs. The time argument
has the same format as offset, except that you
cannot use a leading minus (-) or plus (+) sign.
If time is not specified, the default is 02:00:00.
If no rule is present in the TZ specification, the
rules used are those specified by the tzfile format
file defined by the SYS$POSIXRULES system logical
in the system time-conversion information directory,
with the standard and summer time offsets from UTC
replaced by those specified by the offset values in
TZ.
If TZ does not specify a tzfile format file and
cannot be interpreted as a direct specification, UTC
is used.
NOTE
The UTC-based time functions, introduced in OpenVMS Version
7.0, had degraded performance compared with the non-UTC-
based time functions.
OpenVMS Version 7.1 added a cache for time-zone files to
improve performance. The size of the cache is determined
by the logical name DECC$TZ_CACHE_SIZE. To accommodate most
countries changing the time twice per year, the default
cache size is large enough to hold two time-zone files.
See also ctime, localtime, mktime, strftime, and wcsftime.
2 – Sample TZ Specification
EST5EDT4,M4.1.0,M10.5.0
This sample TZ specification describes the rule defined in 1987
for the Eastern time zone in the US:
o EST (Eastern Standard Time) is the designation for standard
time, which is 5 hours behind UTC.
o EDT (Eastern Daylight Time) is the designation for summer
time, which is 4 hours behind UTC. EDT starts on the first
Sunday in April and ends on the last Sunday in October.
Because time was not specified in either case, the changes
occur at the default time, which is 2:00 A.M. The start and
end dates did not need to be specified, because they are the
defaults.