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    1 NEWTZSET(3)                Library Functions Manual                NEWTZSET(3)
    2 
    3 NAME
    4        tzset - initialize time conversion information
    5 
    6 SYNOPSIS
    7        #include <time.h>
    8 
    9        timezone_t tzalloc(char const *TZ);
   10 
   11        void tzfree(timezone_t tz);
   12 
   13        void tzset(void);
   14 
   15        cc ... -ltz
   16 
   17 DESCRIPTION
   18        Tzalloc allocates and returns a timezone object described by TZ.  If TZ
   19        is not a valid timezone description, or if the object cannot be
   20        allocated, tzalloc returns a null pointer and sets errno.
   21 
   22        Tzfree frees a timezone object tz, which should have been successfully
   23        allocated by tzalloc.  This invalidates any tm_zone pointers that tz
   24        was used to set.
   25 
   26        Tzset acts like tzalloc(getenv("TZ")), except it saves any resulting
   27        timezone object into internal storage that is accessed by localtime,
   28        localtime_r, and mktime.  The anonymous shared timezone object is freed
   29        by the next call to tzset.  If the implied call to tzalloc fails, tzset
   30        falls back on Universal Time (UT).
   31 
   32        If TZ is null, the best available approximation to local wall clock
   33        time, as specified by the tzfile(5)-format file localtime in the system
   34        time conversion information directory, is used.  If TZ is the empty
   35        string, UT is used, with the abbreviation "UTC" and without leap second
   36        correction; please see newctime(3) for more about UT, UTC, and leap
   37        seconds.  If TZ is nonnull and nonempty:
   38 
   39               if the value begins with a colon, it is used as a pathname of a
   40               file from which to read the time conversion information;
   41 
   42               if the value does not begin with a colon, it is first used as
   43               the pathname of a file from which to read the time conversion
   44               information, and, if that file cannot be read, is used directly
   45               as a specification of the time conversion information.
   46 
   47        When TZ is used as a pathname, if it begins with a slash, it is used as
   48        an absolute pathname; otherwise, it is used as a pathname relative to a
   49        system time conversion information directory.  The file must be in the
   50        format specified in tzfile(5).
   51 
   52        When TZ is used directly as a specification of the time conversion
   53        information, it must have the following syntax (spaces inserted for
   54        clarity):
   55 
   56               stdoffset[dst[offset][,rule]]
   57 
   58        Where:
   59 
   60               std and dst    Three or more bytes that are the designation for
   61                              the standard (std) or the alternative (dst, such
   62                              as daylight saving time) time zone.  Only std is
   63                              required; if dst is missing, then daylight saving
   64                              time does not apply in this locale.  Upper- and
   65                              lowercase letters are explicitly allowed.  Any
   66                              characters except a leading colon (:), digits,
   67                              comma (,), ASCII minus (-), ASCII plus (+), and
   68                              NUL bytes are allowed.  Alternatively, a
   69                              designation can be surrounded by angle brackets <
   70                              and >; in this case, the designation can contain
   71                              any characters other than > and NUL.
   72 
   73               offset         Indicates the value one must add to the local
   74                              time to arrive at Coordinated Universal Time.
   75                              The offset has the form:
   76 
   77                                     hh[:mm[:ss]]
   78 
   79                              The minutes (mm) and seconds (ss) are optional.
   80                              The hour (hh) is required and may be a single
   81                              digit.  The offset following std is required.  If
   82                              no offset follows dst, daylight saving time is
   83                              assumed to be one hour ahead of standard time.
   84                              One or more digits may be used; the value is
   85                              always interpreted as a decimal number.  The hour
   86                              must be between zero and 24, and the minutes (and
   87                              seconds) - if present - between zero and 59.  If
   88                              preceded by a "-", the time zone shall be east of
   89                              the Prime Meridian; otherwise it shall be west
   90                              (which may be indicated by an optional preceding
   91                              "+".
   92 
   93               rule           Indicates when to change to and back from
   94                              daylight saving time.  The rule has the form:
   95 
   96                                     date/time,date/time
   97 
   98                              where the first date describes when the change
   99                              from standard to daylight saving time occurs and
  100                              the second date describes when the change back
  101                              happens.  Each time field describes when, in
  102                              current local time, the change to the other time
  103                              is made.  As an extension to POSIX, daylight
  104                              saving is assumed to be in effect all year if it
  105                              begins January 1 at 00:00 and ends December 31 at
  106                              24:00 plus the difference between daylight saving
  107                              and standard time, leaving no room for standard
  108                              time in the calendar.
  109 
  110                              The format of date is one of the following:
  111 
  112                              Jn        The Julian day n (1 <= n <= 365).  Leap
  113                                        days are not counted; that is, in all
  114                                        years - including leap years - February
  115                                        28 is day 59 and March 1 is day 60.  It
  116                                        is impossible to explicitly refer to
  117                                        the occasional February 29.
  118 
  119                              n         The zero-based Julian day
  120                                        (0 <= n <= 365).  Leap days are
  121                                        counted, and it is possible to refer to
  122                                        February 29.
  123 
  124                              Mm.n.d    The d'th day (0 <= d <= 6) of week n of
  125                                        month m of the year (1 <= n <= 5,
  126                                        1 <= m <= 12, where week 5 means "the
  127                                        last d day in month m" which may occur
  128                                        in either the fourth or the fifth
  129                                        week).  Week 1 is the first week in
  130                                        which the d'th day occurs.  Day zero is
  131                                        Sunday.
  132 
  133                              The time has the same format as offset except
  134                              that POSIX does not allow a leading sign ("-" or
  135                              "+").  As an extension to POSIX, the hours part
  136                              of time can range from -167 through 167; this
  137                              allows for unusual rules such as "the Saturday
  138                              before the first Sunday of March".  The default,
  139                              if time is not given, is 02:00:00.
  140 
  141        Here are some examples of TZ values that directly specify the timezone;
  142        they use some of the extensions to POSIX.
  143 
  144        EST5   stands for US Eastern Standard Time (EST), 5 hours behind UT,
  145               without daylight saving.
  146 
  147        <+12>-12<+13>,M11.1.0,M1.2.1/147
  148               stands for Fiji time, 12 hours ahead of UT, springing forward on
  149               November's first Sunday at 02:00, and falling back on January's
  150               second Monday at 147:00 (i.e., 03:00 on the first Sunday on or
  151               after January 14).  The abbreviations for standard and daylight
  152               saving time are "+12" and "+13".
  153 
  154        IST-2IDT,M3.4.4/26,M10.5.0
  155               stands for Israel Standard Time (IST) and Israel Daylight Time
  156               (IDT), 2 hours ahead of UT, springing forward on March's fourth
  157               Thursday at 26:00 (i.e., 02:00 on the first Friday on or after
  158               March 23), and falling back on October's last Sunday at 02:00.
  159 
  160        <-04>4<-03>,J1/0,J365/25
  161               stands for permanent daylight saving time, 3 hours behind UT
  162               with abbreviation "-03".  There is a dummy fall-back transition
  163               on December 31 at 25:00 daylight saving time (i.e., 24:00
  164               standard time, equivalent to January 1 at 00:00 standard time),
  165               and a simultaneous spring-forward transition on January 1 at
  166               00:00 standard time, so daylight saving time is in effect all
  167               year and the initial <-04> is a placeholder.
  168 
  169        <-03>3<-02>,M3.5.0/-2,M10.5.0/-1
  170               stands for time in western Greenland, 3 hours behind UT, where
  171               clocks follow the EU rules of springing forward on March's last
  172               Sunday at 01:00 UT (-02:00 local time, i.e., 22:00 the previous
  173               day) and falling back on October's last Sunday at 01:00 UT
  174               (-01:00 local time, i.e., 23:00 the previous day).  The
  175               abbreviations for standard and daylight saving time are "-03"
  176               and "-02".
  177 
  178        If no rule is present in TZ, the rules specified by the
  179        tzfile(5)-format file posixrules in the system time conversion
  180        information directory are used, with the standard and daylight saving
  181        time offsets from UT replaced by those specified by the offset values
  182        in TZ.
  183 
  184        For compatibility with System V Release 3.1, a semicolon (;) may be
  185        used to separate the rule from the rest of the specification.
  186 
  187 FILES
  188        /usr/share/zoneinfo             timezone information directory
  189        /usr/share/zoneinfo/localtime   local timezone file
  190        /usr/share/zoneinfo/posixrules  used with POSIX-style TZ's
  191        /usr/share/zoneinfo/GMT         for UTC leap seconds
  192 
  193        If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from
  194        /usr/share/zoneinfo/posixrules.
  195 
  196 SEE ALSO
  197        getenv(3), newctime(3), newstrftime(3), time(2), tzfile(5)
  198 
  199                                                                    NEWTZSET(3)