oasis-root

strftime.3p (28556B)

'\" et
.TH STRFTIME "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
strftime,
strftime_l
\(em convert date and time to a string
.SH SYNOPSIS
.LP
.nf
#include <time.h>
.P
size_t strftime(char *restrict \fIs\fP, size_t \fImaxsize\fP,
    const char *restrict \fIformat\fP, const struct tm *restrict \fItimeptr\fP);
size_t strftime_l(char *restrict \fIs\fP, size_t \fImaxsize\fP,
    const char *restrict \fIformat\fP, const struct tm *restrict \fItimeptr\fP,
    locale_t \fIlocale\fP);
.fi
.SH DESCRIPTION
For
\fIstrftime\fR():
The functionality described on this reference page is aligned with the
ISO\ C standard. Any conflict between the requirements described here and the
ISO\ C standard is unintentional. This volume of POSIX.1\(hy2017 defers to the ISO\ C standard.
.P
The
\fIstrftime\fR()
function shall place bytes into the array pointed to by
.IR s
as controlled by the string pointed to by
.IR format .
The format is a character string, beginning and ending in its initial
shift state, if any. The format string consists of zero or more conversion
specifications and ordinary characters.
.P
Each conversion specification is introduced by the
.BR '%' 
character after which the following appear in sequence:
.IP " *" 4
An optional flag:
.RS 4 
.IP "\fR0\fR" 6
The zero character (\c
.BR '0' ),
which specifies that the character used as the padding character is
.BR '0' ,
.IP "\fR+\fR" 6
The
<plus-sign>
character (\c
.BR '+' ),
which specifies that the character used as the padding character is
.BR '0' ,
and that if and only if the field being produced consumes more than four
bytes to represent a year (for
.BR %F ,
.BR %G ,
or
.BR %Y )
or more than two bytes to represent the year divided by 100 (for
.BR %C )
then a leading
<plus-sign>
character shall be included if the year being processed is greater than
or equal to zero or a leading
<hyphen-minus>
character (\c
.BR '\-' )
shall be included if the year is less than zero.
.P
The default padding character is unspecified.
.RE
.IP " *" 4
An optional minimum field width. If the converted value, including
any leading
.BR '+' 
or
.BR '\-' 
sign, has fewer bytes than the minimum field width and the padding
character is not the NUL character, the output shall be padded on the left
(after any leading
.BR '+' 
or
.BR '\-' 
sign) with the padding character.
.IP " *" 4
An optional
.BR E
or
.BR O
modifier.
.IP " *" 4
A terminating conversion specifier character that indicates the type of
conversion to be applied.
.P
The results are unspecified if more than one flag character is specified,
a flag character is specified without a minimum field width; a minimum
field width is specified without a flag character; a modifier is specified
with a flag or with a minimum field width; or if a minimum field width
is specified for any conversion specifier other than
.BR C ,
.BR F ,
.BR G ,
or
.BR Y .
.P
All ordinary characters (including the terminating NUL character)
are copied unchanged into the array. If copying takes place between
objects that overlap, the behavior is undefined. No more than maxsize
bytes are placed into the array. Each conversion specifier is replaced by
appropriate characters as described in the following list. The appropriate
characters are determined using the
.IR LC_TIME
category of the current locale and by the values of zero or more members
of the broken-down time structure pointed to by
.IR timeptr ,
as specified in brackets in the description. If any of the specified
values are outside the normal range, the characters stored are
unspecified.
.P
The
\fIstrftime_l\fR()
function shall be equivalent to the
\fIstrftime\fR()
function, except that the locale data used is from the
locale represented by
.IR locale .
.P
Local timezone information is used as though
\fIstrftime\fR()
called
\fItzset\fR().
.P
The following conversion specifiers shall be supported:
.IP "\fRa\fR" 8
Replaced by the locale's abbreviated weekday name. [\c
.IR tm_wday ]
.IP "\fRA\fR" 8
Replaced by the locale's full weekday name. [\c
.IR tm_wday ]
.IP "\fRb\fR" 8
Replaced by the locale's abbreviated month name. [\c
.IR tm_mon ]
.IP "\fRB\fR" 8
Replaced by the locale's full month name. [\c
.IR tm_mon ]
.IP "\fRc\fR" 8
Replaced by the locale's appropriate date and time representation.
(See the Base Definitions volume of POSIX.1\(hy2017,
.IR <time.h> .)
.IP "\fRC\fR" 8
Replaced by the year divided by 100 and truncated to an integer,
as a decimal number. [\c
.IR tm_year ]
.RS 8 
.P
If a minimum field width is not specified, the number of characters
placed into the array pointed to by
.IR s
will be the number of digits in the year divided by 100 or two, whichever
is greater.
If a minimum field width is specified, the number of characters placed
into the array pointed to by
.IR s
will be the number of digits in the year divided by 100 or the minimum
field width, whichever is greater.
.RE
.IP "\fRd\fR" 8
Replaced by the day of the month as a decimal number [01,31]. [\c
.IR tm_mday ]
.IP "\fRD\fR" 8
Equivalent to
.BR %m /\c
.BR %d /\c
.BR %y .
[\c
.IR tm_mon ,
.IR tm_mday ,
.IR tm_year ]
.IP "\fRe\fR" 8
Replaced by the day of the month as a decimal number [1,31];
a single digit is preceded by a space. [\c
.IR tm_mday ]
.IP "\fRF\fR" 8
Equivalent to %\*!+4\*?Y-%m-%d if no flag and no minimum field width
are specified. [\c
.IR tm_year ,
.IR tm_mon ,
.IR tm_mday ]
.RS 8 
.P
If a minimum field width of
.IR x
is specified, the year shall be output as if by the
.BR Y
specifier (described below) with whatever flag was given and a minimum
field width of
.IR x \-6.
If
.IR x
is less than 6, the behavior shall be as if
.IR x
equalled 6.
.P
If the minimum field width is specified to be 10, and the year is
four digits long, then the output string produced will match the
ISO\ 8601:\|2004 standard subclause 4.1.2.2 complete representation, extended format date
representation of a specific day. If a + flag is specified, a minimum
field width of
.IR x
is specified, and
.IR x \-7
bytes are sufficient to hold the digits of the year (not including any
needed sign character), then the output will match the ISO\ 8601:\|2004 standard subclause
4.1.2.4 complete representation, expanded format date representation of
a specific day.
.RE
.IP "\fRg\fR" 8
Replaced by the last 2 digits of the week-based year (see below)
as a decimal number [00,99]. [\c
.IR tm_year ,
.IR tm_wday ,
.IR tm_yday ]
.IP "\fRG\fR" 8
Replaced by the week-based year (see below) as a decimal number
(for example, 1977). [\c
.IR tm_year ,
.IR tm_wday ,
.IR tm_yday ]
.RS 8 
.P
If a minimum field width is specified, the number of characters placed
into the array pointed to by
.IR s
will be the number of digits and leading sign characters (if any) in
the year, or the minimum field width, whichever is greater.
.RE
.IP "\fRh\fR" 8
Equivalent to
.BR %b .
[\c
.IR tm_mon ]
.IP "\fRH\fR" 8
Replaced by the hour (24-hour clock) as a decimal number [00,23]. [\c
.IR tm_hour ]
.IP "\fRI\fR" 8
Replaced by the hour (12-hour clock) as a decimal number [01,12]. [\c
.IR tm_hour ]
.IP "\fRj\fR" 8
Replaced by the day of the year as a decimal number [001,366]. [\c
.IR tm_yday ]
.IP "\fRm\fR" 8
Replaced by the month as a decimal number [01,12]. [\c
.IR tm_mon ]
.IP "\fRM\fR" 8
Replaced by the minute as a decimal number [00,59]. [\c
.IR tm_min ]
.IP "\fRn\fR" 8
Replaced by a
<newline>.
.IP "\fRp\fR" 8
Replaced by the locale's equivalent of either a.m. or p.m. [\c
.IR tm_hour ]
.IP "\fRr\fR" 8
Replaced by the time in a.m. and p.m. notation;
in the POSIX locale this shall be equivalent to
.BR %I :\c
.BR %M :\c
.BR %S
.BR %p .
[\c
.IR tm_hour ,
.IR tm_min ,
.IR tm_sec ]
.IP "\fRR\fR" 8
Replaced by the time in 24-hour notation (\c
.BR %H :\c
.BR %M ).
[\c
.IR tm_hour ,
.IR tm_min ]
.IP "\fRS\fR" 8
Replaced by the second as a decimal number [00,60]. [\c
.IR tm_sec ]
.IP "\fRt\fR" 8
Replaced by a
<tab>.
.IP "\fRT\fR" 8
Replaced by the time (\c
.BR %H :\c
.BR %M :\c
.BR %S ).
[\c
.IR tm_hour ,
.IR tm_min ,
.IR tm_sec ]
.IP "\fRu\fR" 8
Replaced by the weekday as a decimal number [1,7], with 1 representing
Monday. [\c
.IR tm_wday ]
.IP "\fRU\fR" 8
Replaced by the week number of the year as a decimal number [00,53].
The first Sunday of January is the first day of week 1; days in the
new year before this are in week 0. [\c
.IR tm_year ,
.IR tm_wday ,
.IR tm_yday ]
.IP "\fRV\fR" 8
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 1 January
has four or more days in the new year, then it is considered week 1.
Otherwise, it is the last week of the previous year, and the next week
is week 1. Both January 4th and the first Thursday of January are
always in week 1. [\c
.IR tm_year ,
.IR tm_wday ,
.IR tm_yday ]
.IP "\fRw\fR" 8
Replaced by the weekday as a decimal number [0,6], with 0 representing
Sunday. [\c
.IR tm_wday ]
.IP "\fRW\fR" 8
Replaced by the week number of the year as a decimal number [00,53].
The first Monday of January is the first day of week 1; days in the new
year before this are in week 0. [\c
.IR tm_year ,
.IR tm_wday ,
.IR tm_yday ]
.IP "\fRx\fR" 8
Replaced by the locale's appropriate date representation. (See
the Base Definitions volume of POSIX.1\(hy2017,
.IR <time.h> .)
.IP "\fRX\fR" 8
Replaced by the locale's appropriate time representation. (See
the Base Definitions volume of POSIX.1\(hy2017,
.IR <time.h> .)
.IP "\fRy\fR" 8
Replaced by the last two digits of the year as a decimal number
[00,99]. [\c
.IR tm_year ]
.IP "\fRY\fR" 8
Replaced by the year as a decimal number (for example, 1997). [\c
.IR tm_year ]
.RS 8 
.P
If a minimum field width is specified, the number of characters placed
into the array pointed to by
.IR s
will be the number of digits and leading sign characters (if any) in
the year, or the minimum field width, whichever is greater.
.RE
.IP "\fRz\fR" 8
Replaced by the offset from UTC in the ISO\ 8601:\|2004 standard format (\c
.BR +hhmm
or
.BR \-hhmm ),
or by no characters if no timezone is determinable. For example,
.BR \(dq-0430\(dq 
means 4 hours 30 minutes behind UTC (west of Greenwich).
If
.IR tm_isdst
is zero, the standard time offset is used. If
.IR tm_isdst
is greater than zero, the daylight savings time offset is used. If
.IR tm_isdst
is negative, no characters are returned.
[\c
.IR tm_isdst ]
.IP "\fRZ\fR" 8
Replaced by the timezone name or abbreviation, or by no bytes if no
timezone information exists. [\c
.IR tm_isdst ]
.IP "\fR%\fR" 8
Replaced by
.BR % .
.P
If a conversion specification does not correspond to any of the above,
the behavior is undefined.
.P
If a
.BR "struct tm"
broken-down time structure is created by
\fIlocaltime\fR()
or
\fIlocaltime_r\fR(),
or modified by
\fImktime\fR(),
and the value of
.IR TZ
is subsequently modified, the results of the
.BR %Z
and
.BR %z
\fIstrftime\fR()
conversion specifiers are undefined, when
\fIstrftime\fR()
is called with such a broken-down time structure.
.P
If a
.BR "struct tm"
broken-down time structure is created or modified by
\fIgmtime\fR()
or
\fIgmtime_r\fR(),
it is unspecified whether the result of the
.BR %Z
and
.BR %z
conversion specifiers shall refer to UTC or the current local timezone,
when
\fIstrftime\fR()
is called with such a broken-down time structure.
.SS "Modified Conversion Specifiers"
.P
Some conversion specifiers can be modified by the
.BR E
or
.BR O
modifier characters to indicate that an alternative format or
specification should be used rather than the one normally used by the
unmodified conversion specifier. If the alternative format or
specification does not exist for the current locale (see ERA in the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 7.3.5" ", " "LC_TIME"),
the behavior shall be as if the unmodified conversion
specification were used.
.IP "\fR%Ec\fR" 8
Replaced by the locale's alternative appropriate date and time
representation.
.IP "\fR%EC\fR" 8
Replaced by the name of the base year (period) in the locale's
alternative representation.
.IP "\fR%Ex\fR" 8
Replaced by the locale's alternative date representation.
.IP "\fR%EX\fR" 8
Replaced by the locale's alternative time representation.
.IP "\fR%Ey\fR" 8
Replaced by the offset from
.BR %EC
(year only) in the locale's alternative representation.
.IP "\fR%EY\fR" 8
Replaced by the full alternative year representation.
.IP "\fR%Od\fR" 8
Replaced by the day of the month, using the locale's alternative
numeric symbols, filled as needed with leading zeros if there is any
alternative symbol for zero; otherwise, with leading
<space>
characters.
.IP "\fR%Oe\fR" 8
Replaced by the day of the month, using the locale's alternative
numeric symbols, filled as needed with leading
<space>
characters.
.IP "\fR%OH\fR" 8
Replaced by the hour (24-hour clock) using the locale's alternative
numeric symbols.
.IP "\fR%OI\fR" 8
Replaced by the hour (12-hour clock) using the locale's alternative
numeric symbols.
.IP "\fR%Om\fR" 8
Replaced by the month using the locale's alternative numeric symbols.
.IP "\fR%OM\fR" 8
Replaced by the minutes using the locale's alternative numeric symbols.
.IP "\fR%OS\fR" 8
Replaced by the seconds using the locale's alternative numeric symbols.
.IP "\fR%Ou\fR" 8
Replaced by the weekday as a number in the locale's alternative
representation (Monday=1).
.IP "\fR%OU\fR" 8
Replaced by the week number of the year (Sunday as the first day of the
week, rules corresponding to
.BR %U )
using the locale's alternative numeric symbols.
.IP "\fR%OV\fR" 8
Replaced by the week number of the year (Monday as the first day of the
week, rules corresponding to
.BR %V )
using the locale's alternative numeric symbols.
.IP "\fR%Ow\fR" 8
Replaced by the number of the weekday (Sunday=0) using the locale's
alternative numeric symbols.
.IP "\fR%OW\fR" 8
Replaced by the week number of the year (Monday as the first day of the
week) using the locale's alternative numeric symbols.
.IP "\fR%Oy\fR" 8
Replaced by the year (offset from
.BR %C )
using the locale's alternative numeric symbols.
.P
.BR %g ,
.BR %G ,
and
.BR %V
give values according to the ISO\ 8601:\|2004 standard week-based year. In this system,
weeks begin on a Monday and week 1 of the year is the week that
includes January 4th, which is also the week that includes the first
Thursday of the year, and is also the first week that contains at least
four days in the year. If the first Monday of January is the 2nd, 3rd,
or 4th, the preceding days are part of the last week of the preceding
year; thus, for Saturday 2nd January 1999,
.BR %G
is replaced by 1998 and
.BR %V
is replaced by 53. If December 29th, 30th, or 31st is a Monday, it and
any following days are part of week 1 of the following year. Thus, for
Tuesday 30th December 1997,
.BR %G
is replaced by 1998 and
.BR %V
is replaced by 01.
.P
If a conversion specifier is not one of the above, the behavior is
undefined.
.P
The behavior is undefined if the
.IR locale
argument to
\fIstrftime_l\fR()
is the special locale object LC_GLOBAL_LOCALE or is not a valid locale
object handle.
.SH "RETURN VALUE"
If the total number of resulting bytes including the terminating null
byte is not more than
.IR maxsize ,
these functions shall return the number of bytes placed into the array
pointed to by
.IR s ,
not including the terminating NUL character. Otherwise, 0 shall be returned
and the contents of the array are unspecified.
.SH ERRORS
No errors are defined.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Getting a Localized Date String"
.P
The following example first sets the locale to the user's default. The
locale information will be used in the
\fInl_langinfo\fR()
and
\fIstrftime\fR()
functions. The
\fInl_langinfo\fR()
function returns the localized date string which specifies how the date
is laid out. The
\fIstrftime\fR()
function takes this information and, using the
.BR tm
structure for values, places the date and time information into
.IR datestring .
.sp
.RS 4
.nf
#include <time.h>
#include <locale.h>
#include <langinfo.h>
\&...
struct tm *tm;
char datestring[256];
\&...
setlocale (LC_ALL, "");
\&...
strftime (datestring, sizeof(datestring), nl_langinfo (D_T_FMT), tm);
\&...
.fi
.P
.RE
.SH "APPLICATION USAGE"
The range of values for
.BR %S
is [00,60] rather than [00,59] to allow for the occasional leap
second.
.P
Some of the conversion specifications are duplicates of others. They
are included for compatibility with
\fInl_cxtime\fR()
and
\fInl_ascxtime\fR(),
which were published in Issue 2.
.P
The
.BR %C ,
.BR %F ,
.BR %G ,
and
.BR %Y
format specifiers in
\fIstrftime\fR()
always print full values, but the
\fIstrptime\fR()
.BR %C ,
.BR %F ,
and
.BR %Y
format specifiers only scan two digits (assumed to be the first two
digits of a four-digit year) for
.BR %C
and four digits (assumed to be the entire (four-digit) year) for
.BR %F
and
.BR %Y .
This mimics the behavior of
\fIprintf\fR()
and
\fIscanf\fR();
that is:
.sp
.RS 4
.nf
printf("%2d", x = 1000);
.fi
.P
.RE
.P
prints
.BR \(dq1000\(dq ,
but:
.sp
.RS 4
.nf
scanf(%2d", &x);
.fi
.P
.RE
.P
when given
.BR \(dq1000\(dq 
as input will only store 10 in
.IR x ).
Applications using extended ranges of years must be sure that the number
of digits specified for scanning years with
\fIstrptime\fR()
matches the number of digits that will actually be present in the input
stream. Historic implementations of the
.BR %Y
conversion specification (with no flags and no minimum field width)
produced different output formats. Some always produced at least four
digits (with 0 fill for years from 0 through 999) while others only
produced the number of digits present in the year (with no fill and no
padding). These two forms can be produced with the
.BR '0' 
flag and a minimum field width options using the conversions
specifications
.BR %04Y
and
.BR %01Y ,
respectively.
.P
In the past, the C and POSIX standards specified that
.BR %F
produced an ISO\ 8601:\|2004 standard date format, but didn't specify which one. For
years in the range [0001,9999], POSIX.1\(hy2008 requires that the output
produced match the ISO\ 8601:\|2004 standard complete representation extended format
(YYYY-MM-DD) and for years outside of this range produce output
that matches the ISO\ 8601:\|2004 standard expanded representation extended format
(<+/-><Underline>Y</Underline>YYYY-MM-DD). To fully meet ISO\ 8601:\|2004 standard
requirements, the producer and consumer must agree on a date format that
has a specific number of bytes reserved to hold the characters used to
represent the years that is sufficiently large to hold all values that
will be shared. For example, the
.BR %+13F
conversion specification will produce output matching the format
.BR \(dq<+/->YYYYYY-MM-DD\(dq 
(a leading
.BR '+' 
or
.BR '\-' 
sign; a six-digit, 0-filled year; a
.BR '\-' ;
a two-digit, leading 0-filled month; another
.BR '\-' ;
and the two-digit, leading 0-filled day within the month).
.P
Note that if the year being printed is greater than 9999, the resulting
string from the unadorned
.BR %F
conversion specifications will not conform to the ISO\ 8601:\|2004 standard extended format,
complete representation for a date and will instead be an extended format,
expanded representation (presumably without the required agreement
between the date's producer and consumer).
.P
In the C or POSIX locale, the
.BR E
and
.BR O
modifiers are ignored and the replacement strings for the following
specifiers are:
.IP "\fR%a\fR" 8
The first three characters of
.BR %A .
.IP "\fR%A\fR" 8
One of Sunday, Monday, .\|.\|., Saturday.
.IP "\fR%b\fR" 8
The first three characters of
.BR %B .
.IP "\fR%B\fR" 8
One of January, February, .\|.\|., December.
.IP "\fR%c\fR" 8
Equivalent to
.BR %a
.BR %b
.BR %e
.BR %T
.BR %Y .
.IP "\fR%p\fR" 8
One of AM or PM.
.IP "\fR%r\fR" 8
Equivalent to
.BR %I :\c
.BR %M :\c
.BR %S
.BR %p .
.IP "\fR%x\fR" 8
Equivalent to
.BR %m /\c
.BR %d /\c
.BR %y .
.IP "\fR%X\fR" 8
Equivalent to
.BR %T .
.IP "\fR%Z\fR" 8
Implementation-defined.
.SH RATIONALE
The
.BR %Y
conversion specification to
\fIstrftime\fR()
was frequently assumed to be a four-digit year, but the ISO\ C standard does not
specify that
.BR %Y
is restricted to any subset of allowed values from the
.IR tm_year
field. Similarly, the
.BR %C
conversion specification was assumed to be a two-digit field and the
first part of the output from the
.BR %F
conversion specification was assumed to be a four-digit field. With
.IR tm_year
being a signed 32 or more-bit
.BR int
and with many current implementations supporting 64-bit
.BR time_t
types in one or more programming environments, these assumptions are
clearly wrong.
.P
POSIX.1\(hy2008 now allows the format specifications
.BR %0xC ,
.BR %0xF ,
.BR %0xG ,
and
.BR %0xY
(where
.BR 'x' 
is a string of decimal digits used to specify printing and scanning of a
string of
.IR x
decimal digits) with leading zero fill characters. Allowing applications
to set the field width enables them to agree on the number of digits
to be printed and scanned in the ISO\ 8601:\|2004 standard expanded representation of a
year (for
.BR %F ,
.BR %G ,
and
.BR %Y )
or all but the last two digits of the year (for
.BR %C ).
This is based on a feature in some versions of GNU
.BR libc 's
\fIstrftime\fR().
The GNU version allows specifying space, zero, or no-fill characters in
\fIstrftime\fR()
format strings, but does not allow any flags to be specified in
\fIstrptime\fR()
format strings. These implementations also allow these flags to be
specified for any numeric field. POSIX.1\(hy2008 only requires the zero fill flag
(\c
.BR '0' )
and only requires that it be recognized when processing
.BR %C ,
.BR %F ,
.BR %G ,
and
.BR %Y
specifications when a minimum field width is also specified. The
.BR '0' 
flag is the only flag needed to produce and scan the ISO\ 8601:\|2004 standard
year fields using the extended format forms. POSIX.1\(hy2008 also allows
applications to specify the same flag and field width specifiers to be
used in both
\fIstrftime\fR()
and
\fIstrptime\fR()
format strings for symmetry. Systems may provide other flag characters
and may accept flags in conjunction with conversion specifiers other than
.BR %C ,
.BR %F ,
.BR %G ,
and
.BR %Y ;
but portable applications cannot depend on such extensions.
.P
POSIX.1\(hy2008 now also allows the format specifications
.BR %+xC ,
.BR %+xF ,
.BR %+xG ,
and
.BR %+xY
(where
.BR 'x' 
is a string of decimal digits used to specify printing and scanning of
a string of
.BR 'x' 
decimal digits) with leading zero fill characters and a leading
.BR '+' 
sign character if the year being converted is more than four digits
or a minimum field width is specified that allows room for more than
four digits for the year. This allows date providers and consumers to
agree on a specific number of digits to represent a year as required by
the ISO\ 8601:\|2004 standard expanded representation formats. The expanded representation
formats all require the year to begin with a leading
.BR '+' 
or
.BR '\-' 
sign.
(All of these specifiers can also provide a leading
.BR '\-' 
sign for negative years. Since negative years and the year 0 don't fit
well with the Gregorian or Julian calendars, the normal ranges of dates
start with year 1. The ISO\ C standard allows
.IR tm_year
to assume values corresponding to years before year 1, but the use of
such years provided unspecified results.)
.P
Some earlier version of this standard specified that applications wanting
to use
\fIstrptime\fR()
to scan dates and times printed by
\fIstrftime\fR()
should provide non-digit characters between fields to separate years
from months and days. It also supported
.BR %F
to print and scan the ISO\ 8601:\|2004 standard extended format, complete representation date
for years 1 through 9999 (i.e., YYYY-MM-DD). However, many applications
were written to print (using
\fIstrftime\fR())
and scan (using
\fIstrptime\fR())
dates written using the basic format complete representation
(four-digit years) and truncated representation (two-digit years)
specified by the ISO\ 8601:\|2004 standard representation of dates and times which do not
have any separation characters between fields. The ISO\ 8601:\|2004 standard also specifies
basic format expanded representation where the creator and consumer of
these fields agree beforehand to represent years as leading zero-filled
strings of an agreed length of more than four digits to represent a year
(again with no separation characters when year, month, and day are all
displayed). Applications producing and consuming expanded representations
are encouraged to use the
.BR '+' 
flag and an appropriate maximum field width to scan the year including
the leading sign. Note that even without the
.BR '+' 
flag, years less than zero may be represented with a leading
<hyphen-minus>
for
.BR %F ,
.BR %G ,
and
.BR %Y
conversion specifications. Using negative years results in unspecified
behavior.
.P
If a format specification
.BR %+xF
with the field width
.IR x
greater than 11 is specified and the width is large enough to display
the full year, the output string produced will match the ISO\ 8601:\|2004 standard
subclause 4.1.2.4 expanded representation, extended format date
representation for a specific day. (For years in the range [1,99\|999],
.BR %+12F
is sufficient for an agreed five-digit year with a leading sign using
the ISO\ 8601:\|2004 standard expanded representation, extended format for a specific day
.BR \(dq<+/->YYYYY-MM-DD\(dq .)
Note also that years less than 0 may produce a leading
<hyphen-minus>
character (\c
.BR '\-' )
when using
.BR %Y
or
.BR %C
whether or not the
.BR '0' 
or
.BR '+' 
flags are used.
.P
The difference between the
.BR '0' 
flag and the
.BR '+' 
flag is whether the leading
.BR '+' 
character will be provided for years >9999 as required for the ISO\ 8601:\|2004 standard
extended representation format containing a year. For example:
.TS
box center tab(!);
cB | cB | cB | cB
cB | cB | cB | cB
l | lf5 | l | l.
!!\fIstrftime\fP(\^)!\fIstrptime\fP(\^)
Year!Conversion Specification!Output!Scan Back
_
1970!%Y!1970!1970
_
1970!%+4Y!1970!1970
_
27!%Y!27 or 0027!27
_
270!%Y!270 or 0270!270
_
270!%+4Y!0270!270
_
17!%C%y!0017!17
_
270!%C%y!0270!270
_
12345!%Y!12345!1234*
_
12345!%+4Y!+12345!123*
_
12345!%05Y!12345!12345
_
270!%+5Y \fRor\fP %+3C%y!+0270!270
_
12345!%+5Y \fRor\fP %+3C%y!+12345!1234*
_
12345!%06Y \fRor\fP %04C%y!012345!12345
_
12345!%+6Y \fRor\fP %+4C%y!+12345!12345
_
123456!%08Y \fRor\fP %06C%y!00123456!123456
_
123456!%+8Y \fRor\fP %+6C%y!+0123456!123456
.TE
.P
In the cases above marked with a * in the
\fIstrptime\fR()
scan back field, the implied or specified number of characters scanned by
\fIstrptime\fR()
was less than the number of characters output by
\fIstrftime\fR()
using the same format; so the remaining digits of the year were dropped
when the output date produced by
\fIstrftime\fR()
was scanned back in by
\fIstrptime\fR().
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIasctime\fR\^(\|)",
.IR "\fIclock\fR\^(\|)",
.IR "\fIctime\fR\^(\|)",
.IR "\fIdifftime\fR\^(\|)",
.IR "\fIgetdate\fR\^(\|)",
.IR "\fIgmtime\fR\^(\|)",
.IR "\fIlocaltime\fR\^(\|)",
.IR "\fImktime\fR\^(\|)",
.IR "\fIstrptime\fR\^(\|)",
.IR "\fItime\fR\^(\|)",
.IR "\fItzset\fR\^(\|)",
.IR "\fIuselocale\fR\^(\|)",
.IR "\fIutime\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 7.3.5" ", " "LC_TIME",
.IR "\fB<time.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .