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.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
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.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sat Jul 24 19:49:27 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified Fri Apr 26 12:38:55 MET DST 1996 by Martin Schulze (joey@linux.de)
.\" Modified 2001-11-13, aeb
.\" Modified 2001-12-13, joey, aeb
.\" Modified 2004-11-16, mtk
.\"
.TH CTIME 3  2008-09-26 "" "Linux Programmer's Manual"
.SH NAME
asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r,
localtime_r \- transform date and time to broken-down time or ASCII
.SH SYNOPSIS
.nf
.B #include <time.h>
.sp
.BI "char *asctime(const struct tm *" tm );
.br
.BI "char *asctime_r(const struct tm *" tm ", char *" buf );
.sp
.BI "char *ctime(const time_t *" timep );
.br
.BI "char *ctime_r(const time_t *" timep ", char *" buf );
.sp
.BI "struct tm *gmtime(const time_t *" timep );
.br
.BI "struct tm *gmtime_r(const time_t *" timep ", struct tm *" result );
.sp
.BI "struct tm *localtime(const time_t *" timep );
.br
.BI "struct tm *localtime_r(const time_t *" timep ", struct tm *" result );
.sp
.BI "time_t mktime(struct tm *" tm );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR asctime_r (),
.BR ctime_r (),
.BR gmtime_r (),
.BR localtime_r ():
.br
_POSIX_C_SOURCE\ >=\ 1 || _XOPEN_SOURCE || _BSD_SOURCE ||
_SVID_SOURCE || _POSIX_SOURCE
.SH DESCRIPTION
The
.BR ctime (),
.BR gmtime ()
and
.BR localtime ()
functions all take
an argument of data type \fItime_t\fP which represents calendar time.
When interpreted as an absolute time value, it represents the number of
seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal
Time (UTC).
.PP
The
.BR asctime ()
and
.BR mktime ()
functions both take an argument
representing broken-down time which is a representation
separated into year, month, day, etc.
.PP
Broken-down time is stored
in the structure \fItm\fP which is defined in \fI<time.h>\fP as follows:
.sp
.in +4n
.nf
struct tm {
    int tm_sec;         /* seconds */
    int tm_min;         /* minutes */
    int tm_hour;        /* hours */
    int tm_mday;        /* day of the month */
    int tm_mon;         /* month */
    int tm_year;        /* year */
    int tm_wday;        /* day of the week */
    int tm_yday;        /* day in the year */
    int tm_isdst;       /* daylight saving time */
};
.fi
.in
.PP
The members of the \fItm\fP structure are:
.TP 10
.I tm_sec
The number of seconds after the minute, normally in the range 0 to 59,
but can be up to 60 to allow for leap seconds.
.TP
.I tm_min
The number of minutes after the hour, in the range 0 to 59.
.TP
.I tm_hour
The number of hours past midnight, in the range 0 to 23.
.TP
.I tm_mday
The day of the month, in the range 1 to 31.
.TP
.I tm_mon
The number of months since January, in the range 0 to 11.
.TP
.I tm_year
The number of years since 1900.
.TP
.I tm_wday
The number of days since Sunday, in the range 0 to 6.
.TP
.I tm_yday
The number of days since January 1, in the range 0 to 365.
.TP
.I tm_isdst
A flag that indicates whether daylight saving time is in effect at the
time described.
The value is positive if daylight saving time is in
effect, zero if it is not, and negative if the information is not
available.
.PP
The call
.BI ctime( t )
is equivalent to
.BI asctime(localtime( t )) \fR.
It converts the calendar time \fIt\fP into a
null-terminated string of the form
.sp
.RS
"Wed Jun 30 21:49:08 1993\\n"
.RE
.sp
The abbreviations for the days of the week are "Sun", "Mon", "Tue", "Wed",
"Thu", "Fri", and "Sat".
The abbreviations for the months are "Jan",
"Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", and
"Dec".
The return value points to a statically allocated string which
might be overwritten by subsequent calls to any of the date and time
functions.
The function also sets the external variable \fItzname\fP (see
.BR tzset (3))
with information about the current timezone.
The reentrant version
.BR ctime_r ()
does the same, but stores the
string in a user-supplied buffer
which should have room for at least 26 bytes.
It need not
set \fItzname\fP.
.PP
The
.BR gmtime ()
function converts the calendar time \fItimep\fP to
broken-down time representation, expressed in Coordinated Universal Time
(UTC).
It may return NULL when the year does not fit into an integer.
The return value points to a statically allocated struct which might be
overwritten by subsequent calls to any of the date and time functions.
The
.BR gmtime_r ()
function does the same, but stores the data in a
user-supplied struct.
.PP
The
.BR localtime ()
function converts the calendar time \fItimep\fP to
broken-time representation, expressed relative to the user's specified
timezone.
The function acts as if it called
.BR tzset (3)
and sets the external variables \fItzname\fP with
information about the current timezone, \fItimezone\fP with the difference
between Coordinated Universal Time (UTC) and local standard time in
seconds, and \fIdaylight\fP to a non-zero value if daylight savings
time rules apply during some part of the year.
The return value points to a statically allocated struct which might be
overwritten by subsequent calls to any of the date and time functions.
The
.BR localtime_r ()
function does the same, but stores the data in a
user-supplied struct.
It need not set \fItzname\fP.
.PP
The
.BR asctime ()
function converts the broken-down time value
\fItm\fP into a null-terminated string with the same format as
.BR ctime ().
The return value points to a statically allocated string which might be
overwritten by subsequent calls to any of the date and time functions.
The
.BR asctime_r ()
function does the same, but stores the string in
a user-supplied buffer which should have room for at least 26 bytes.
.PP
The
.BR mktime ()
function converts a broken-down time structure, expressed
as local time, to calendar time representation.
The function ignores
the values supplied by the caller in the
.I tm_wday
and
.I tm_yday
fields.
The value specified in the
.I tm_isdst
field informs
.BR mktime ()
whether or not daylight saving time (DST)
is in effect for the time supplied in the
.I tm
structure:
a positive value means DST is in effect;
zero means that DST is not in effect;
and a negative value means that
.BR mktime ()
should (use timezone information and system databases to)
attempt to determine whether DST is in effect at the specified time.

The
.BR mktime ()
function modifies the fields of the
.IR tm
structure as follows:
.I tm_wday
and
.I tm_yday
are set to values determined from the contents of the other fields;
if structure members are outside their valid interval, they will be
normalized (so that, for example, 40 October is changed into 9 November);
.I tm_isdst
is set (regardless of its initial value)
to a positive value or to 0, respectively,
to indicate whether DST is or is not in effect at the specified time. 
Calling
.BR mktime ()
also sets the external variable \fItzname\fP with
information about the current timezone.

If the specified broken-down
time cannot be represented as calendar time (seconds since the Epoch),
.BR mktime ()
returns a value of
.I (time_t)\ \-1
and does not alter the
members of the broken-down time structure.
.SH "RETURN VALUE"
Each of these functions returns the value described, or NULL
(\-1 in case of
.BR mktime ())
in case an error was detected.
.SH "CONFORMING TO"
POSIX.1-2001.
C89 and C99 specify
.BR asctime (),
.BR ctime (),
.BR gmtime (),
.BR localtime (),
and
.BR mktime ().
POSIX.1-2008 marks
.BR asctime (),
.BR asctime_r (),
.BR ctime (),
and
.BR ctime_r ()
as obsolete.
.SH NOTES
The four functions
.BR asctime (),
.BR ctime (),
.BR gmtime ()
and
.BR localtime ()
return a pointer to static data and hence are not thread-safe.
Thread-safe versions
.BR asctime_r (),
.BR ctime_r (),
.BR gmtime_r ()
and
.BR localtime_r ()
are specified by SUSv2, and available since libc 5.2.5.

POSIX.1-2001 says:
"The
.BR asctime (),
.BR ctime (),
.BR gmtime (),
and
.BR localtime ()
functions shall return values in one of two static objects:
a broken-down time structure and an array of type
.IR char .
Execution of any of the functions may overwrite the information returned
in either of these objects by any of the other functions."
This can occur in the glibc implementation.
.LP
In many implementations, including glibc, a 0 in
.I tm_mday
is interpreted as meaning the last day of the preceding month.
.LP
The glibc version of \fIstruct tm\fP has additional fields
.sp
.RS
.nf
long tm_gmtoff;           /* Seconds east of UTC */
const char *tm_zone;      /* Timezone abbreviation */
.fi
.RE
.sp
defined when
.B _BSD_SOURCE
was set before including
.IR <time.h> .
This is a BSD extension, present in 4.3BSD-Reno.

According to POSIX.1-2004,
.BR localtime ()
is required to behave as though
.BR tzset ()
was called, while
.BR localtime_r ()
does not have this requirement.
.\" See http://thread.gmane.org/gmane.comp.time.tz/2034/
For portable code
.BR tzset ()
should be called before
.BR localtime_r ().
.SH "SEE ALSO"
.BR date (1),
.BR gettimeofday (2),
.BR time (2),
.BR utime (2),
.BR clock (3),
.BR difftime (3),
.BR strftime (3),
.BR strptime (3),
.BR timegm (3),
.BR tzset (3),
.BR time (7)