STRFTIME(3P) POSIX Programmer's Manual STRFTIME(3P)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.
NAME
strftime, strftime_l — convert date and time to a string
SYNOPSIS
#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);
DESCRIPTION
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 specifica‐
tions 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 out‐
put 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 speci‐
fied, 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 appro‐
priate 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() func‐
tion, 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 representa‐
tion. (See the Base Definitions volume of POSIX.1‐2008,
<time.h>.)
C Replaced by the year divided by 100 and truncated to an inte‐
ger, as a decimal number. [tm_year]
If a minimum field width is not specified, the number of char‐
acters 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 represen‐
tation, extended format date representation of a specific day.
If a + flag is specified, a minimum field width of x is speci‐
fied, 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 com‐
plete 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 rep‐
resenting 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 con‐
taining 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 rep‐
resenting 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 stan‐
dard 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 subse‐
quently 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 struc‐
ture.
Modified Conversion Specifiers
Some conversion specifiers can be modified by the E or O modifier char‐
acters 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 alterna‐
tive numeric symbols, filled as needed with leading zeros if
there is any alternative symbol for zero; otherwise, with lead‐
ing <space> characters.
%Oe Replaced by the day of the month, using the locale's alterna‐
tive numeric symbols, filled as needed with leading <space>
characters.
%OH Replaced by the hour (24-hour clock) using the locale's alter‐
native numeric symbols.
%OI Replaced by the hour (12-hour clock) using the locale's alter‐
native 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 sym‐
bols.
%Oy Replaced by the year (offset from %C) using the locale's alter‐
native 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.
RETURN VALUE
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 ter‐
minating NUL character. Otherwise, 0 shall be returned and the contents
of the array are unspecified.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
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);
...
APPLICATION USAGE
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 dig‐
its that will actually be present in the input stream. Historic imple‐
mentations 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 conver‐
sions 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 pro‐
duce output that matches the ISO 8601:2004 standard expanded represen‐
tation extended format (<+/-><Underline>Y</Underline>YYYY-MM-DD). To
fully meet ISO 8601:2004 standard requirements, the producer and con‐
sumer 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 representa‐
tion (presumably without the required agreement between the date's pro‐
ducer 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.
RATIONALE
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 sup‐
porting 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 strf‐
time() 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 pro‐
cessing %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 speci‐
fied 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 Gre‐
gorian 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 want‐
ing 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 sub‐
clause 4.1.2.4 expanded representation, extended format date represen‐
tation 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 out‐
put date produced by strftime() was scanned back in by strptime().
FUTURE DIRECTIONS
None.
SEE ALSOasctime(), clock(), ctime(), difftime(), getdate(), gmtime(), local‐
time(), mktime(), strptime(), time(), tzset(), uselocale(), utime()
The Base Definitions volume of POSIX.1‐2008, Section 7.3.5, LC_TIME,
<time.h>
COPYRIGHT
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 Electri‐
cal 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.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 STRFTIME(3P)
