DESCRIPTION

     gettimeofday and settimeofday can set the time as well as  a
     timezone.   tv   is  a  timeval  struct,  as  specified   in
     /usr/include/sys/time.h:

     struct timeval {
             long tv_sec;        /* seconds */
             long tv_usec;  /* microseconds */
     };

     and tz is a timezone :

     struct timezone {
             int  tz_minuteswest; /* minutes W of Greenwich */
             int  tz_dsttime;     /* type of dst correction */
     };

     The use of the timezone struct is obsolete;  the  tz_dsttime
     field  has never been used under Linux - it has not been and
     will not be supported by libc  or  glibc.   Each  and  every
     occurrence  of  this  field in the kernel source (other than
     the declaration) is a bug. Thus, the following is purely  of
     historic interest.

     The field tz_dsttime contains a  symbolic  constant  (values
     are  given  below)  that indicates in which part of the year
     Daylight Saving Time is in force. (Note: its value  is  con-
     stant throughout the year - it does not indicate that DST is
     in force, it just selects an algorithm.)  The daylight  sav-
     ing time algorithms defined are as follows :

      DST_NONE     /* not on dst */
      DST_USA      /* USA style dst */
      DST_AUST     /* Australian style dst */
      DST_WET      /* Western European dst */
      DST_MET      /* Middle European dst */
      DST_EET      /* Eastern European dst */
      DST_CAN      /* Canada */
      DST_GB       /* Great Britain and Eire */
      DST_RUM      /* Rumania */
      DST_TUR      /* Turkey */
      DST_AUSTALT  /* Australian style with shift in 1986 */

     Of course it turned out that the period  in  which  Daylight
     Saving  Time  is  in force cannot be given by a simple algo-
     rithm, one per country; indeed, this period is determined by
     unpredictable   political   decisions.  So  this  method  of
     representing time zones has been abandoned. Under Linux,  in
     a call to settimeofday the tz_dsttime field should be zero.

     Under Linux there is some peculiar  `warp  clock'  semantics
     associated  to  the  settimeofday system call if on the very
     first call (after booting) that has a non-NULL tz  argument,
     the  tv  argument  is  NULL  and the tz_minuteswest field is
     nonzero. In such a case it is assumed that the CMOS clock is
     on  local  time,  and  that it has to be incremented by this
     amount to get UTC system time.  No doubt it is a bad idea to
     use this feature.

     The following macros are defined  to  operate  on  a  struct
     timeval :
     #define       timerisset(tvp)\
             ((tvp)->tv_sec || (tvp)->tv_usec)
     #define       timercmp(tvp, uvp, cmp)\
             ((tvp)->tv_sec cmp (uvp)->tv_sec ||\
             (tvp)->tv_sec == (uvp)->tv_sec &&\
             (tvp)->tv_usec cmp (uvp)->tv_usec)
     #define       timerclear(tvp)\
             ((tvp)->tv_sec = (tvp)->tv_usec = 0)

     If either tv or tz is null, the corresponding  structure  is
     not set or returned.

     Only the super user may use settimeofday.

RETURN VALUES

     gettimeofday and settimeofday return 0 for  success,  or  -1
     for failure (in which case errno is set appropriately).

ERRORS

     EPERM
          settimeofday  is  called  by  someone  other  than  the
          superuser.

     EINVAL
          Timezone (or something else) is invalid.

     EFAULT
          One of tv or tz pointed outside your accessible address
          space.

CONFORMING TO

     SVr4, BSD 4.3

SEE ALSO

     date(1), adjtimex(2), time(2),