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.
