.\" Copyright (c) 1989, 1991 The Regents of the University of California.
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.\"     from: @(#)strftime.3    5.12 (Berkeley) 6/29/91
.\"	$OpenBSD: strftime.3,v 1.36 2016/02/08 20:29:11 jmc Exp $
.\"
.Dd $Mdocdate: February 8 2016 $
.Dt STRFTIME 3
.Os
.Sh NAME
.Nm strftime
.Nd format date and time
.Sh SYNOPSIS
.In time.h
.Ft size_t
.Fn strftime "char *buf" "size_t maxsize" "const char *format" "const struct tm *timeptr"
.Sh DESCRIPTION
The
.Fn strftime
function formats the information from
.Fa timeptr
(as described in
.Xr mktime 3 )
into the buffer
.Fa buf
according to the string pointed to by
.Fa format .
.Pp
The
.Fa format
string consists of zero or more conversion specifications and
ordinary characters.
All ordinary characters are copied directly into the buffer.
A conversion specification consists of a percent sign
.Ql %
and one other character.
.Pp
No more than
.Fa maxsize
characters will be placed into the array.
.Pp
Each conversion specification is replaced by the characters as
follows which are then copied into the buffer.
.Bl -tag -width "xxxx"
.It Cm \&%A
is replaced by the locale's full weekday name.
.It Cm \&%a
is replaced by the locale's abbreviated weekday name.
.It Cm \&%B
is replaced by the locale's full month name.
.It Cm \&%b No or Cm \&%h
is replaced by the locale's abbreviated month name.
.It Cm \&%C
is replaced by the century (a year divided by 100 and truncated to an integer)
as a decimal number (00\-99).
.It Cm \&%c
is replaced by the locale's appropriate date and time representation.
.It Cm \&%D
is replaced by the date in the format
.Dq Li %m/%d/%y .
.It Cm \&%d
is replaced by the day of the month as a decimal number (01\-31).
.It Cm \&%e
is replaced by the day of month as a decimal number (1\-31);
single digits are preceded by a blank.
.It Cm \&%F
is replaced by the date in the format
.Dq Li %Y-%m-%d .
.It Cm \&%G
is replaced by the
.St -iso8601
year with century as a decimal number.
.It Cm \&%g
is replaced by the
.St -iso8601
year without century as a decimal number (00\-99).
.It Cm \&%H
is replaced by the hour (24-hour clock) as a decimal number (00\-23).
.It Cm \&%I
is replaced by the hour (12-hour clock) as a decimal number (01\-12).
.It Cm \&%j
is replaced by the day of the year as a decimal number (001\-366).
.It Cm \&%k
is replaced by the hour (24-hour clock) as a decimal number (0\-23);
single digits are preceded by a blank.
.It Cm \&%l
is replaced by the hour (12-hour clock) as a decimal number (1\-12);
single digits are preceded by a blank.
.It Cm \&%M
is replaced by the minute as a decimal number (00\-59).
.It Cm %m
is replaced by the month as a decimal number (01\-12).
.It Cm %n
is replaced by a newline.
.It Cm %p
is replaced by the locale's equivalent of either
.Dq AM
or
.Dq PM .
.It Cm \&%R
is replaced by the time in the format
.Dq Li %H:%M .
.It Cm \&%r
is replaced by the locale's representation of 12-hour clock time
using AM/PM notation.
.It Cm \&%S
is replaced by the second as a decimal number (00\-60).
The range of
seconds is (00\-60) instead of (00\-59) to allow for the periodic occurrence
of leap seconds.
.It Cm %s
is replaced by the number of seconds since the Epoch, UTC (see
.Xr mktime 3 ) .
.It Cm \&%T
is replaced by the time in the format
.Dq Li %H:%M:%S .
.It Cm \&%t
is replaced by a tab.
.It Cm \&%U
is replaced by the week number of the year (Sunday as the first day of
the week) as a decimal number (00\-53).
.It Cm \&%u
is replaced by the weekday (Monday as the first day of the week)
as a decimal number (1\-7).
.It Cm \&%V
is replaced by the week number of the year (Monday as the first day of
the week) as a decimal number (01\-53).
If the week containing January
1 has four or more days in the new year, then it is week 1; otherwise
it is week 53 of the previous year, and the next week is week 1.
.It Cm \&%v
is replaced by the date in the format
.Dq Li "%e-%b-%Y" .
.It Cm \&%W
is replaced by the week number of the year (Monday as the first day of
the week) as a decimal number (00\-53).
.It Cm \&%w
is replaced by the weekday (Sunday as the first day of the week)
as a decimal number (0\-6).
.It Cm \&%X
is replaced by the locale's appropriate time representation.
.It Cm \&%x
is replaced by the locale's appropriate date representation.
.It Cm \&%Y
is replaced by the year with century as a decimal number.
.It Cm \&%y
is replaced by the year without century as a decimal number (00\-99).
.It Cm \&%Z
is replaced by the time zone name,
or by the empty string if this is not determinable.
.It Cm \&%z
is replaced by the offset from UTC in the format
.Dq Li +HHMM
or
.Dq Li -HHMM
as appropriate, with positive values representing locations east
of Greenwich, or by the empty string if this is not determinable.
.It Cm %%
is replaced by
.Ql % .
.It Cm %+
is replaced by the date and time in
.Xr date 1
format.
.El
.Sh RETURN VALUES
If the total number of resulting characters, including the terminating
NUL character, is not more than
.Fa maxsize ,
.Fn strftime
returns the number of characters placed in the array, not counting the
terminating NUL.
Otherwise, zero is returned.
.Sh SEE ALSO
.Xr date 1 ,
.Xr printf 1 ,
.Xr ctime 3 ,
.Xr getenv 3 ,
.Xr printf 3 ,
.Xr strptime 3 ,
.Xr time 3 ,
.Xr tzset 3 ,
.Xr tzfile 5
.Sh STANDARDS
The
.Fn strftime
function
conforms to
.St -isoC-99 ,
except that the
.Ql E
and
.Ql O
conversion modifiers are ignored by this implementation.
.Pp
The
.Ql \&%k ,
.Ql \&%l ,
.Ql \&%s ,
.Ql \&%v ,
and
.Ql \&%+
conversion specifications are extensions.
.Pp
Use of the
.St -iso8601
conversions may produce non-intuitive results.
Week 01 of a year is per definition the first week which has the Thursday
in this year, which is equivalent to the week which contains the fourth
day of January.
In other words, the first week of a new year is the week which has the
majority of its days in the new year.
Week 01 might also contain days from the previous year and the week
before week 01 of a year is the last week (52 or 53) of the previous
year even if it contains days from the new year.
A week starts with Monday (day 1) and ends with Sunday (day 7).
For example, the first week of the year 1997 lasts from
1996-12-30 to 1997-01-05.
.Sh HISTORY
A predecessor to
.Fn strftime ,
.Fn ptime ,
first appeared in
.At v1 .
The
.Fn strftime
function has been available since
.Bx 4.3 Reno .
.Sh AUTHORS
.An Keith Bostic
implemented the
.Bx
version of
.Fn strftime
in 1989.
.Sh BUGS
There is no conversion specification for the phase of the moon.
.Pp
Note that while this implementation of
.Fn strftime
will always NUL terminate
.Fa buf ,
other implementations may not do so when
.Fa maxsize
is not large enough to store the entire time string.
The contents of
.Fa buf
are implementation specific in this case.