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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.1‐2017 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 <hyphen-minus> 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.1‐2017, <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.1‐2017, <time.h>.)
X Replaced by the locale's appropriate time representation. (See the Base
Definitions volume of POSIX.1‐2017, <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.1‐2017, 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.
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 terminating 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 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 or POSIX 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 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 <hyphen-minus> 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 <hyphen-minus> character ('-') 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().
FUTURE DIRECTIONS
None.
SEE ALSO
asctime(), clock(), ctime(), difftime(), getdate(), gmtime(), localtime(), mktime(),
strptime(), time(), tzset(), uselocale(), utime()
The Base Definitions volume of POSIX.1‐2017, Section 7.3.5, LC_TIME, <time.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, 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 .
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 .