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STRFTIME(3P) POSIX Programmer's Manual STRFTIME(3P)
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.
strftime, strftime_l — convert date and time to a string
#include <time.h>
size_t strftime(char *restrict s, size_t maxsize,
const char *restrict format, const struct tm *restrict timeptr);
size_t strftime_l(char *restrict s, size_t maxsize,
const char *restrict format, const struct tm *restrict timeptr,
locale_t locale);
For strftime(): 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.1‐2008 defers to the ISO C standard.
The strftime() function shall place bytes into the array pointed to
by s as controlled by the string pointed to by 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.
Each conversion specification is introduced by the '%' character
after which the following appear in sequence:
* An optional flag:
0 The zero character ('0'), which specifies that the
character used as the padding character is '0',
+ The <plus-sign> character ('+'), which specifies that the
character used as the padding character is '0', and that if
and only if the field being produced consumes more than
four bytes to represent a year (for %F, %G, or %Y) or more
than two bytes to represent the year divided by 100 (for
%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 minus-sign character ('−') shall be
included if the year is less than zero.
The default padding character is unspecified.
* An optional minimum field width. If the converted value,
including any leading '+' or '−' 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 '+' or '−' sign) with the padding character.
* An optional E or O modifier.
* A terminating conversion specifier character that indicates the
type of conversion to be applied.
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 C, F, G, or Y.
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 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 timeptr, as
specified in brackets in the description. If any of the specified
values are outside the normal range, the characters stored are
unspecified.
The strftime_l() function shall be equivalent to the strftime()
function, except that the locale data used is from the locale
represented by locale.
Local timezone information is used as though strftime() called
tzset().
The following conversion specifiers shall be supported:
a Replaced by the locale's abbreviated weekday name. [tm_wday]
A Replaced by the locale's full weekday name. [tm_wday]
b Replaced by the locale's abbreviated month name. [tm_mon]
B Replaced by the locale's full month name. [tm_mon]
c Replaced by the locale's appropriate date and time
representation. (See the Base Definitions volume of
POSIX.1‐2008, <time.h>.)
C Replaced by the year divided by 100 and truncated to an
integer, as a decimal number. [tm_year]
If a minimum field width is not specified, the number of
characters placed into the array pointed to by 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 s
will be the number of digits in the year divided by 100 or
the minimum field width, whichever is greater.
d Replaced by the day of the month as a decimal number [01,31].
[tm_mday]
D Equivalent to %m/%d/%y. [tm_mon, tm_mday, tm_year]
e Replaced by the day of the month as a decimal number [1,31];
a single digit is preceded by a space. [tm_mday]
F Equivalent to %+4Y-%m-%d if no flag and no minimum field
width are specified. [tm_year, tm_mon, tm_mday]
If a minimum field width of x is specified, the year shall be
output as if by the Y specifier (described below) with
whatever flag was given and a minimum field width of x−6. If
x is less than 6, the behavior shall be as if x equalled 6.
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 x is specified, and 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.
g Replaced by the last 2 digits of the week-based year (see
below) as a decimal number [00,99]. [tm_year, tm_wday,
tm_yday]
G Replaced by the week-based year (see below) as a decimal
number (for example, 1977). [tm_year, tm_wday, tm_yday]
If a minimum field width is specified, the number of
characters placed into the array pointed to by s will be the
number of digits and leading sign characters (if any) in the
year, or the minimum field width, whichever is greater.
h Equivalent to %b. [tm_mon]
H Replaced by the hour (24-hour clock) as a decimal number
[00,23]. [tm_hour]
I Replaced by the hour (12-hour clock) as a decimal number
[01,12]. [tm_hour]
j Replaced by the day of the year as a decimal number
[001,366]. [tm_yday]
m Replaced by the month as a decimal number [01,12]. [tm_mon]
M Replaced by the minute as a decimal number [00,59]. [tm_min]
n Replaced by a <newline>.
p Replaced by the locale's equivalent of either a.m. or p.m.
[tm_hour]
r Replaced by the time in a.m. and p.m. notation; in the POSIX
locale this shall be equivalent to %I:%M:%S %p. [tm_hour,
tm_min, tm_sec]
R Replaced by the time in 24-hour notation (%H:%M). [tm_hour,
tm_min]
S Replaced by the second as a decimal number [00,60]. [tm_sec]
t Replaced by a <tab>.
T Replaced by the time (%H:%M:%S). [tm_hour, tm_min, tm_sec]
u Replaced by the weekday as a decimal number [1,7], with 1
representing Monday. [tm_wday]
U 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.
[tm_year, tm_wday, tm_yday]
V 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. [tm_year, tm_wday, tm_yday]
w Replaced by the weekday as a decimal number [0,6], with 0
representing Sunday. [tm_wday]
W 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.
[tm_year, tm_wday, tm_yday]
x Replaced by the locale's appropriate date representation.
(See the Base Definitions volume of POSIX.1‐2008, <time.h>.)
X Replaced by the locale's appropriate time representation.
(See the Base Definitions volume of POSIX.1‐2008, <time.h>.)
y Replaced by the last two digits of the year as a decimal
number [00,99]. [tm_year]
Y Replaced by the year as a decimal number (for example, 1997).
[tm_year]
If a minimum field width is specified, the number of
characters placed into the array pointed to by s will be the
number of digits and leading sign characters (if any) in the
year, or the minimum field width, whichever is greater.
z Replaced by the offset from UTC in the ISO 8601:2004 standard
format (+hhmm or −hhmm), or by no characters if no timezone
is determinable. For example, "−0430" means 4 hours 30
minutes behind UTC (west of Greenwich). If tm_isdst is zero,
the standard time offset is used. If tm_isdst is greater than
zero, the daylight savings time offset is used. If tm_isdst
is negative, no characters are returned. [tm_isdst]
Z Replaced by the timezone name or abbreviation, or by no bytes
if no timezone information exists. [tm_isdst]
% Replaced by %.
If a conversion specification does not correspond to any of the
above, the behavior is undefined.
If a struct tm broken-down time structure is created by localtime()
or localtime_r(), or modified by mktime(), and the value of TZ is
subsequently modified, the results of the %Z and %z strftime()
conversion specifiers are undefined, when strftime() is called with
such a broken-down time structure.
If a struct tm broken-down time structure is created or modified by
gmtime() or gmtime_r(), it is unspecified whether the result of the
%Z and %z conversion specifiers shall refer to UTC or the current
local timezone, when strftime() is called with such a broken-down
time structure.
Modified Conversion Specifiers
Some conversion specifiers can be modified by the E or 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.1‐2008, Section 7.3.5, LC_TIME), the behavior shall
be as if the unmodified conversion specification were used.
%Ec Replaced by the locale's alternative appropriate date and
time representation.
%EC Replaced by the name of the base year (period) in the
locale's alternative representation.
%Ex Replaced by the locale's alternative date representation.
%EX Replaced by the locale's alternative time representation.
%Ey Replaced by the offset from %EC (year only) in the locale's
alternative representation.
%EY Replaced by the full alternative year representation.
%Od 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.
%Oe Replaced by the day of the month, using the locale's
alternative numeric symbols, filled as needed with leading
<space> characters.
%OH Replaced by the hour (24-hour clock) using the locale's
alternative numeric symbols.
%OI Replaced by the hour (12-hour clock) using the locale's
alternative numeric symbols.
%Om Replaced by the month using the locale's alternative numeric
symbols.
%OM Replaced by the minutes using the locale's alternative
numeric symbols.
%OS Replaced by the seconds using the locale's alternative
numeric symbols.
%Ou Replaced by the weekday as a number in the locale's
alternative representation (Monday=1).
%OU Replaced by the week number of the year (Sunday as the first
day of the week, rules corresponding to %U) using the
locale's alternative numeric symbols.
%OV Replaced by the week number of the year (Monday as the first
day of the week, rules corresponding to %V) using the
locale's alternative numeric symbols.
%Ow Replaced by the number of the weekday (Sunday=0) using the
locale's alternative numeric symbols.
%OW Replaced by the week number of the year (Monday as the first
day of the week) using the locale's alternative numeric
symbols.
%Oy Replaced by the year (offset from %C) using the locale's
alternative numeric symbols.
%g, %G, and %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, %G is replaced by 1998 and %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, %G is replaced by 1998 and %V is replaced by 01.
If a conversion specifier is not one of the above, the behavior is
undefined.
The behavior is undefined if the locale argument to strftime_l() is
the special locale object LC_GLOBAL_LOCALE or is not a valid locale
object handle.
If the total number of resulting bytes including the terminating null
byte is not more than maxsize, these functions shall return the
number of bytes placed into the array pointed to by s, not including
the terminating NUL character. Otherwise, 0 shall be returned and the
contents of the array are unspecified.
No errors are defined.
The following sections are informative.
Getting a Localized Date String
The following example first sets the locale to the user's default.
The locale information will be used in the nl_langinfo() and
strftime() functions. The nl_langinfo() function returns the
localized date string which specifies how the date is laid out. The
strftime() function takes this information and, using the tm
structure for values, places the date and time information into
datestring.
#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);
...
The range of values for %S is [00,60] rather than [00,59] to allow
for the occasional leap second.
Some of the conversion specifications are duplicates of others. They
are included for compatibility with nl_cxtime() and nl_ascxtime(),
which were published in Issue 2.
The %C, %F, %G, and %Y format specifiers in strftime() always print
full values, but the strptime() %C, %F, and %Y format specifiers only
scan two digits (assumed to be the first two digits of a four-digit
year) for %C and four digits (assumed to be the entire (four-digit)
year) for %F and %Y. This mimics the behavior of printf() and
scanf(); that is:
printf("%2d", x = 1000);
prints "1000", but:
scanf(%2d", &x);
when given "1000" as input will only store 10 in x). Applications
using extended ranges of years must be sure that the number of digits
specified for scanning years with strptime() matches the number of
digits that will actually be present in the input stream. Historic
implementations of the %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 '0' flag and a minimum field width options using
the conversions specifications %04Y and %01Y, respectively.
In the past, the C and POSIX standards specified that %F produced an
ISO 8601:2004 standard date format, but didn't specify which one. For
years in the range [0001,9999], POSIX.1‐2008 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 %+13F conversion specification will produce output
matching the format "<+/->YYYYYY-MM-DD" (a leading '+' or '−' sign; a
six-digit, 0-filled year; a '−'; a two-digit, leading 0-filled month;
another '−'; and the two-digit, leading 0-filled day within the
month).
Note that if the year being printed is greater than 9999, the
resulting string from the unadorned %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).
In the C locale, the E and O modifiers are ignored and the
replacement strings for the following specifiers are:
%a The first three characters of %A.
%A One of Sunday, Monday, ..., Saturday.
%b The first three characters of %B.
%B One of January, February, ..., December.
%c Equivalent to %a %b %e %T %Y.
%p One of AM or PM.
%r Equivalent to %I:%M:%S %p.
%x Equivalent to %m/%d/%y.
%X Equivalent to %T.
%Z Implementation-defined.
The %Y conversion specification to strftime() was frequently assumed
to be a four-digit year, but the ISO C standard does not specify that
%Y is restricted to any subset of allowed values from the tm_year
field. Similarly, the %C conversion specification was assumed to be a
two-digit field and the first part of the output from the %F
conversion specification was assumed to be a four-digit field. With
tm_year being a signed 32 or more-bit int and with many current
implementations supporting 64-bit time_t types in one or more
programming environments, these assumptions are clearly wrong.
POSIX.1‐2008 now allows the format specifications %0xC, %0xF, %0xG,
and %0xY (where 'x' is a string of decimal digits used to specify
printing and scanning of a string of 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 %F, %G, and %Y) or all but the last two digits of the year
(for %C). This is based on a feature in some versions of GNU libc's
strftime(). The GNU version allows specifying space, zero, or no-
fill characters in strftime() format strings, but does not allow any
flags to be specified in strptime() format strings. These
implementations also allow these flags to be specified for any
numeric field. POSIX.1‐2008 only requires the zero fill flag ('0')
and only requires that it be recognized when processing %C, %F, %G,
and %Y specifications when a minimum field width is also specified.
The '0' flag is the only flag needed to produce and scan the
ISO 8601:2004 standard year fields using the extended format forms.
POSIX.1‐2008 also allows applications to specify the same flag and
field width specifiers to be used in both strftime() and strptime()
format strings for symmetry. Systems may provide other flag
characters and may accept flags in conjunction with conversion
specifiers other than %C, %F, %G, and %Y; but portable applications
cannot depend on such extensions.
POSIX.1‐2008 now also allows the format specifications %+xC, %+xF,
%+xG, and %+xY (where 'x' is a string of decimal digits used to
specify printing and scanning of a string of 'x' decimal digits) with
leading zero fill characters and a leading '+' 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 '+' or '−' sign.
(All of these specifiers can also provide a leading '−' 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 tm_year to assume values
corresponding to years before year 1, but the use of such years
provided unspecified results.)
Some earlier version of this standard specified that applications
wanting to use strptime() to scan dates and times printed by
strftime() should provide non-digit characters between fields to
separate years from months and days. It also supported %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 strftime())
and scan (using strptime()) 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 '+' flag
and an appropriate maximum field width to scan the year including the
leading sign. Note that even without the '+' flag, years less than
zero may be represented with a leading minus-sign for %F, %G,and %Y
conversion specifications. Using negative years results in
unspecified behavior.
If a format specification %+xF with the field width 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 [1,99999],
%+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 "<+/->YYYYY-MM-DD".) Note also that years
less than 0 may produce a leading minus-sign ('−') when using %Y or
%C whether or not the '0' or '+' flags are used.
The difference between the '0' flag and the '+' flag is whether the
leading '+' character will be provided for years >9999 as required
for the ISO 8601:2004 standard extended representation format
containing a year. For example:
┌───────┬──────────────────────────┬─────────────┬────────────┐
│ │ │ strftime() │ strptime() │
│ 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 or %+3C%y │ +0270 │ 270 │
├───────┼──────────────────────────┼─────────────┼────────────┤
│12345 │ %+5Y or %+3C%y │ +12345 │ 1234* │
├───────┼──────────────────────────┼─────────────┼────────────┤
│12345 │ %06Y or %04C%y │ 012345 │ 12345 │
├───────┼──────────────────────────┼─────────────┼────────────┤
│12345 │ %+6Y or %+4C%y │ +12345 │ 12345 │
├───────┼──────────────────────────┼─────────────┼────────────┤
│123456 │ %08Y or %06C%y │ 00123456 │ 123456 │
├───────┼──────────────────────────┼─────────────┼────────────┤
│123456 │ %+8Y or %+6C%y │ +0123456 │ 123456 │
└───────┴──────────────────────────┴─────────────┴────────────┘
In the cases above marked with a * in the strptime() scan back field,
the implied or specified number of characters scanned by strptime()
was less than the number of characters output by strftime() using the
same format; so the remaining digits of the year were dropped when
the output date produced by strftime() was scanned back in by
strptime().
None.
asctime(3p), clock(3p), ctime(3p), difftime(3p), getdate(3p),
gmtime(3p), localtime(3p), mktime(3p), strptime(3p), time(3p),
tzset(3p), uselocale(3p), utime(3p)
The Base Definitions volume of POSIX.1‐2008, Section 7.3.5, LC_TIME,
time.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
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 .
IEEE/The Open Group 2013 STRFTIME(3P)
Pages that refer to this page: langinfo.h(0p), time.h(0p), date(1p), asctime(3p), clock(3p), ctime(3p), difftime(3p), getdate(3p), gmtime(3p), localeconv(3p), localtime(3p), mktime(3p), strptime(3p), time(3p), tzset(3p), wcsftime(3p)