STRFTIME(3)

NAME
strftime - format date and time

LIBRARY
libc

SYNOPSIS
#include <time.h>
size_t
strftime(char * restrict buf, size_t maxsize, const char * restrict format, const struct tm * restrict timeptr);
size_t
strftime_l(char * restrict buf, size_t maxsize, const char * restrict format, const struct tm * restrict timeptr, locale_t loc);
size_t
strftime_z(const timezone_t tz, char * restrict buf, size_t maxsize, const char * restrict format, const struct tm * restrict timeptr);
size_t
strftime_lz(const timezone_t tz, char * restrict buf, size_t maxsize, const char * restrict format, const struct tm * restrict timeptr, locale_t loc);

DESCRIPTION
The strftime() function formats the information from timeptr into the array pointed to by buf according to the string pointed to by format.

The function strftime_l() does the same as strftime() but takes an explicit locale specified in the locale_t loc argument, rather than using the current locale.

The function strftime_z() is similar to strftime(), but uses an explicit timezone specified in the const timezone_t tz argument, instead of using the default from the environment.

The function strftime_lz() does the same as strftime() but takes both an explicit timezone and locale arguments.

The format string consists of zero or more conversion specifications and ordinary characters. All ordinary characters are copied directly into the array. A conversion specification consists of a percent sign ‘%’ and one other character.

No more than maxsize characters will be placed into the array. If the total number of resulting characters, including the terminating NUL character, is not more than maxsize, strftime() returns the number of characters in the array, not counting the terminating NUL. Otherwise, zero is returned and the buffer contents are indeterminate.

Each conversion specification is replaced by the characters as follows which are then copied into the array. The characters depend on the values of zero or more members of timeptr as specified by brackets in the description. If a bracketed member name is followed by “+”, strftime can use the named member even though POSIX.1-2024 does not list it; if the name is followed by “-”, strftime ignores the member even though POSIX.1-2024 lists it which means portable code should set it. For portability, timeptr should be initialized as if by a successful call to gmtime(3), localtime(3), mktime(3), timegm(3), or similar functions.

%A  is replaced by the locale's full weekday name. [tm_wday]

%a  is replaced by the locale's abbreviated weekday name. [tm_wday]

%B  is replaced by the locale's full month name. [tm_mon]

%b or %h
    is replaced by the locale's abbreviated month name.

%C  is replaced by the century (a year divided by 100 and truncated to an integer) as a decimal number, with at least two digits by default.

%c  is replaced by the locale's appropriate date and time representation. [tm_year, tm_yday, tm_mon, tm_mday, tm_wday, tm_hour, tm_min, tm_sec, tm_gmtoff, tm_zone, tm_isdst -].

%D  is replaced by the date in the format “%m/%d/%y”. Although used in the United States for current dates, this format is ambiguous elsewhere and for dates that might involve other centuries. [tm_year, tm_mon, tm_mday]

%d  is replaced by the day of the month as a decimal number [01,31]. [tm_mday]

%d  is replaced by the day of the month as a decimal number (01-31).

%E* %O*
    POSIX locale extensions. The sequences %Ec %EC %Ex %EX %Ey %EY %Od %Oe %OH %OI %Om %OM %OS %Ou %OU %OV %Ow %OW %Oy are supposed to provide alternate representations.

    Additionally %OB implemented to represent alternative months names (used standalone, without day mentioned).

%e  is replaced by the day of month as a decimal number [1,31]; single digits are preceded by a blank. [tm_mday]

%F  is equivalent to “%Y-%m-%d” (the ISO 8601 date format). [tm_year, tm_mon, tm_mday]

%G  is replaced by the ISO 8601 year with century as a decimal number. See also the %V conversion specification [tm_year, tm_yday, tm_wday]

%g  is replaced by the ISO 8601 year without century as a decimal number. This is the year that includes the greater part of the week. (Monday as the first day of a week). See also the ‘%V’ conversion specification. [tm_year, tm_yday, tm_wday]

%H  is replaced by the hour (24-hour clock) as a decimal number [00,23]. [tm_hour]

%I  is replaced by the hour (12-hour clock) as a decimal number [01,12]. [tm_hour]

%j  is replaced by the day of the year as a decimal number [001,366]. [tm_yday]

%k  is replaced by the hour (24-hour clock) as a decimal number [0,23]; single digits are preceded by a blank. [tm_hour]

%l  is replaced by the hour (12-hour clock) as a decimal number [1,12]; single digits are preceded by a blank. [tm_hour]

%M  is replaced by the minute as a decimal number [00,59]. [tm_min]

%m  is replaced by the month as a decimal number [01,12]. [tm_mon]

%n  is replaced by a newline.

%p  is replaced by the locale's equivalent of either “AM” or “PM”. [tm_hour]

%R  is replaced by the time in the format “%H:%M”. [tm_hour, tm_min]

%r  is replaced by the locale's representation of 12-hour clock time using AM/PM notation.

%S  is replaced by the second as a decimal number [00,60]. The range of seconds is [00-60] instead of [00-59] to allow for the periodic occurrence of leap seconds. [tm_sec]

%s  is replaced by the number of seconds since the Epoch (see ctime(3)). Although %s is reliable in this implementation, it can have glitches on other platforms (notably obsolescent platforms lacking tm_gmtoff or where is no wider than int), and POSIX allows strftime to set errno to EINVAL or EOVERFLOW and return 0 if the number of seconds would be negative or out of range for Portable code should therefore format a value directly via something like snprintf(3) instead of via localtime(3) followed by strftime with "%s". [tm_year, tm_mon, tm_mday, tm_hour, tm_min, tm_sec, tm_gmtoff +, tm_isdst -].

%T  is replaced by the time in the format “%H:%M:%S”. [tm_hour, tm_min, tm_sec]

%t  is replaced by a tab.

%U  is replaced by the week number of the year (Sunday as the first day of the week) as a decimal number [00,53]. [tm_wday, tm_yday, tm_year -]

%u  is replaced by the weekday (Monday as the first day of the week) as a decimal number [1,7]. [tm_wday]

%V  is replaced by the week number of the year (Monday as the first day of the week) as a decimal number [01,53]. According to ISO 8601 the week containing January 1 is week 1 if it has four or more days in the new year, otherwise it is week 53 of the previous year, and the next week is week 1. The year is given by the ‘%G’ conversion specification. [tm_year, tm_yday, tm_wday]

%v  is replaced by the date in the format “%e-%b-%Y”. [tm_year, tm_yday, tm_wday]

%W  is replaced by the week number of the year (Monday as the first day of the week) as a decimal number [00,53]. [tm_yday, tm_wday]

%w  is replaced by the weekday (Sunday as the first day of the week) as a decimal number [0,6]. [tm_year, tm_yday, tm_wday]

%X  is replaced by the locale's appropriate time representation. [tm_year -, tm_yday -, tm_mon -, tm_mday -, tm_wday -, tm_hour, tm_min, tm_sec, tm_gmtoff, tm_zone, tm_isdst -].

%x  is replaced by the locale's appropriate date representation. [tm_year, tm_yday, tm_mon, tm_mday, tm_wday, tm_hour -, tm_min -, tm_sec -, tm_gmtoff -, tm_zone -, tm_isdst -].

%Y  is replaced by the year with century as a decimal number. [tm_year]

%y  is replaced by the year without century as a decimal number [00,99]. [tm_year]

%Z  is replaced by the time zone abbreviation, or the empty string if this is not determinable. [tm_zone, tm_isdst -]

%z  is replaced by the offset from the Prime Meridian in the format +HHMM or -HHMM (ISO 8601) as appropriate, with positive values representing locations east of Greenwich, or by the empty string if this is not determinable. The numeric time zone abbreviation -0000 is used when the time is Universal Time but local time is indeterminate; by convention this is used for locations while uninhabited, and corresponds to a zero offset when the time zone abbreviation begins with “[-]”.

%+  is replaced by locale's date and time in date(1) format. On NetBSD currently this only works for the C locale. [tm_year, tm_yday, tm_mon, tm_mday, tm_wday, tm_hour, tm_min, tm_sec, tm_gmtoff, tm_zone]

%-* GNU libc extension. Do not do any padding when performing numerical outputs.

%_* GNU libc extension. Explicitly specify space for padding.

%0* GNU libc extension. Explicitly specify zero for padding.

%%  is replaced by as single ‘%’.

As a side effect, strftime also behaves as if tzset(3) were called. This is for compatibility with older platforms, as required by POSIX; it is not needed for strftime own use.

RETURN VALUES
If the conversion is successful, strftime returns the number of bytes placed into the array, not counting the terminating NUL; errno is unchanged if the returned value is zero. Otherwise, errno is set to indicate the error, zero is returned, and the array contents are unspecified.

ERRORS
This function fails if:

[ERANGE]
    The specified file offset is invalid. The total number of resulting bytes, including the terminating NUL character, is more than maxsize.

[EOVERFLOW]
    The format includes an %s conversion and the number of seconds since the Epoch cannot be represented in a time_t.

SEE ALSO
date(1), printf(1), ctime(3), printf(3), strptime(3), tm(3)

STANDARDS
The strftime() function conforms to isoC-99. The ‘%C’, ‘%D’, ‘%e’, ‘%g’, ‘%G’, ‘%h’, ‘%k’, ‘%l’, ‘%n’, ‘%r’, ‘%R’, ‘%s’, ‘%t’, ‘%T’, ‘%u’, ‘%V’, and ‘%v’ conversion specifications are extensions.

Use of the ISO 8601 conversions may produce non-intuitive results. Week 01 of a year is per definition the first week which has the Thursday in this year, which is equivalent to the week which contains the fourth day of January. In other words, the first week of a new year is the week which has the majority of its days in the new year. Week 01 might also contain days from the previous year and the week before week 01 of a year is the last week (52 or 53) of the previous year even if it contains days from the new year. A week starts with Monday (day 1) and ends with Sunday (day 7). For example, the first week of the year 1997 lasts from 1996-12-30 to 1997-01-05.

BUGS
There is no conversion specification for the phase of the moon.

The strftime() function does not correctly handle multibyte characters in the format argument.

A return value of zero does not necessarily indicate an error. If the resulting string is an empty string, the result value is zero and it is not possible to distinguish between success and error. For example, in many locales %p yields an empty string. This problem can be avoided by inserting an extra space at the beginning of the format string and then skipping over it or removing it from the result.