.\" This file is in the public domain, so clarified as of

.\" 2009-05-17 by Arthur David Olson.

.TH newctime 3 "" "Time Zone Database"

.SH NAME

asctime, ctime, difftime, gmtime, localtime, mktime \- convert date and time

.SH SYNOPSIS

.nf

.ie \n(.g .ds - \f(CR-\fP

.el .ds - \-

.B #include <time.h>

.PP

.BR "extern char *tzname[];" " /\(** (optional) \(**/"

.PP

.B [[deprecated]] char *ctime(time_t const *clock);

.PP

.B char *ctime_r(time_t const *clock, char *buf);

.PP

.B double difftime(time_t time1, time_t time0);

.PP

.B [[deprecated]] char *asctime(struct tm const *tm);

.PP

.B "char *asctime_r(struct tm const *restrict tm,"

.B " char *restrict result);"

.PP

.B struct tm *localtime(time_t const *clock);

.PP

.B "struct tm *localtime_r(time_t const *restrict clock,"

.B " struct tm *restrict result);"

.PP

.B "struct tm *localtime_rz(timezone_t restrict zone,"

.B " time_t const *restrict clock,"

.B " struct tm *restrict result);"

.PP

.B struct tm *gmtime(time_t const *clock);

.PP

.B "struct tm *gmtime_r(time_t const *restrict clock,"

.B " struct tm *restrict result);"

.PP

.B time_t mktime(struct tm *tm);

.PP

.B "time_t mktime_z(timezone_t restrict zone,"

.B " struct tm *restrict tm);"

.PP

.B cc ... \*-ltz

.fi

.SH DESCRIPTION

.ie '\(en'' .ds en \-

.el .ds en \(en

.ie '\(lq'' .ds lq \&"\"

.el .ds lq \(lq\"

.ie '\(rq'' .ds rq \&"\"

.el .ds rq \(rq\"

.de q

\\$3\*(lq\\$1\*(rq\\$2

..

The

.B ctime

function

converts a long integer, pointed to by

.IR clock ,

and returns a pointer to a

string of the form

.br

.ce

.eo

Thu Nov 24 18:22:48 1986\n\0

.br

.ec

Years requiring fewer than four characters are padded with leading zeroes.

For years longer than four characters, the string is of the form

.br

.ce

.eo

Thu Nov 24 18:22:48 81986\n\0

.ec

.br

with five spaces before the year.

These unusual formats are designed to make it less likely that older

software that expects exactly 26 bytes of output will mistakenly output

misleading values for out-of-range years.

.PP

The

.BI * clock

timestamp represents the time in seconds since 1970-01-01 00:00:00

Coordinated Universal Time (UTC).

The POSIX standard says that timestamps must be nonnegative

and must ignore leap seconds.

Many implementations extend POSIX by allowing negative timestamps,

and can therefore represent timestamps that predate the

introduction of UTC and are some other flavor of Universal Time (UT).

Some implementations support leap seconds, in contradiction to POSIX.

.PP

The

.B ctime

function is deprecated starting in C23.

Callers can use

.B localtime_r

and

.B strftime

instead.

.PP

The

.B localtime

and

.B gmtime

functions

return pointers to

.q "tm"

structures, described below.

The

.B localtime

function

corrects for the time zone and any time zone adjustments

(such as Daylight Saving Time in the United States).

After filling in the

.q "tm"

structure,

.B localtime

sets the

.BR tm_isdst 'th

element of

.B tzname

to a pointer to a string that's the time zone abbreviation to be used with

.BR localtime 's

return value.

.PP

The

.B gmtime

function

converts to Coordinated Universal Time.

.PP

The

.B asctime

function

converts a time value contained in a

.q "tm"

structure to a string,

as shown in the above example,

and returns a pointer to the string.

This function is deprecated starting in C23.

Callers can use

.B strftime

instead.

.PP

The

.B mktime

function

converts the broken-down time,

expressed as local time,

in the structure pointed to by

.I tm

into a calendar time value with the same encoding as that of the values

returned by the

.B time

function.

The original values of the

.B tm_wday

and

.B tm_yday

components of the structure are ignored,

and the original values of the other components are not restricted

to their normal ranges.

(A positive or zero value for

.B tm_isdst

causes

.B mktime

to presume initially that daylight saving time

respectively,

is or is not in effect for the specified time.

A negative value for

.B tm_isdst

causes the

.B mktime

function to attempt to divine whether daylight saving time is in effect

for the specified time; in this case it does not use a consistent

rule and may give a different answer when later

presented with the same argument.)

On successful completion, the values of the

.B tm_wday

and

.B tm_yday

components of the structure are set appropriately,

and the other components are set to represent the specified calendar time,

but with their values forced to their normal ranges; the final value of

.B tm_mday

is not set until

.B tm_mon

and

.B tm_year

are determined.

The

.B mktime

function

returns the specified calendar time;

If the calendar time cannot be represented,

it returns \-1.

.PP

The

.B difftime

function

returns the difference between two calendar times,

.RI ( time1

\-

.IR time0 ),

expressed in seconds.

.PP

The

.BR ctime_r ,

.BR localtime_r ,

.BR gmtime_r ,

and

.B asctime_r

functions

are like their unsuffixed counterparts, except that they accept an

additional argument specifying where to store the result if successful.

.PP

The

.B localtime_rz

and

.B mktime_z

functions

are like their unsuffixed counterparts, except that they accept an

extra initial

.B zone

argument specifying the timezone to be used for conversion.

If

.B zone

is null, UT is used; otherwise,

.B zone

should be have been allocated by

.B tzalloc

and should not be freed until after all uses (e.g., by calls to

.BR strftime )

of the filled-in

.B tm_zone

fields.

.PP

Declarations of all the functions and externals, and the

.q "tm"

structure,

are in the

.B <time.h>

header file.

The structure (of type)

.B struct tm

includes the following fields:

.RS

.PP

.nf

.ta 2n +\w'long tm_gmtoff;nn'u

int tm_sec; /\(** seconds (0\*(en60) \(**/

int tm_min; /\(** minutes (0\*(en59) \(**/

int tm_hour; /\(** hours (0\*(en23) \(**/

int tm_mday; /\(** day of month (1\*(en31) \(**/

int tm_mon; /\(** month of year (0\*(en11) \(**/

int tm_year; /\(** year \- 1900 \(**/

int tm_wday; /\(** day of week (Sunday = 0) \(**/

int tm_yday; /\(** day of year (0\*(en365) \(**/

int tm_isdst; /\(** is daylight saving time in effect? \(**/

char \(**tm_zone; /\(** time zone abbreviation (optional) \(**/

long tm_gmtoff; /\(** offset from UT in seconds (optional) \(**/

.fi

.RE

.PP

The

.B tm_isdst

field

is non-zero if daylight saving time is in effect.

.PP

The

.B tm_gmtoff

field

is the offset (in seconds) of the time represented

from UT, with positive values indicating east

of the Prime Meridian.

The field's name is derived from Greenwich Mean Time, a precursor of UT.

.PP

In

.B "struct tm"

the

.B tm_zone

and

.B tm_gmtoff

fields exist, and are filled in, only if arrangements to do

so were made when the library containing these functions was

created.

Similarly, the

.B tzname

variable is optional; also, there is no guarantee that

.B tzname

will

continue to exist in this form in future releases of this code.

.SH FILES

.ta \w'/usr/share/zoneinfo/posixrules\0\0'u

/usr/share/zoneinfo timezone information directory

.br

/usr/share/zoneinfo/localtime local timezone file

.br

/usr/share/zoneinfo/posixrules default DST rules (obsolete,

and can cause bugs if present)

.br

/usr/share/zoneinfo/GMT for UTC leap seconds

.sp

If

.B /usr/share/zoneinfo/GMT

is absent,

UTC leap seconds are loaded from

.BR /usr/share/zoneinfo/posixrules .

.SH SEE ALSO

getenv(3),

newstrftime(3),

newtzset(3),

time(2),

tzfile(5)

.SH NOTES

The return values of

.BR asctime ,

.BR ctime ,

.BR gmtime ,

and

.B localtime

point to static data

overwritten by each call.

The

.B tzname

variable (once set) and the

.B tm_zone

field of a returned

.B "struct tm"

both point to an array of characters that

can be freed or overwritten by later calls to the functions

.BR localtime ,

.BR tzfree ,

and

.BR tzset ,

if these functions affect the timezone information that specifies the

abbreviation in question.

The remaining functions and data are thread-safe.

.PP

The

.BR asctime ,

.BR asctime_r ,

.BR ctime ,

and

.B ctime_r

functions

behave strangely for years before 1000 or after 9999.

The 1989 and 1999 editions of the C Standard say

that years from \-99 through 999 are converted without

extra spaces, but this conflicts with longstanding

tradition and with this implementation.

The 2011 edition says that the behavior

is undefined if the year is before 1000 or after 9999.

Traditional implementations of these two functions are

restricted to years in the range 1900 through 2099.

To avoid this portability mess, new programs should use

.B strftime

instead.