strftime, strftime_z —
format date and time
Standard C Library (libc, -lc)
#include <time.h>
size_t
  
  strftime(char
    * restrict buf, size_t
    maxsize, const char *
    restrict format, const
    struct tm * restrict timeptr);
size_t
  
  strftime_z(const
    timezone_t tz, char *
    restrict buf, size_t
    maxsize, const char *
    restrict format, const
    struct tm * restrict timeptr);
The strftime() function formats the information from
  timeptr into the array pointed to by
  buf according to the string pointed to by
  format.
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 bytes will be placed
    into the array. Otherwise, zero is returned.
Each conversion specification is replaced by the characters as
    follows which are then copied into the array.
  - %A
- is replaced by the locale's full weekday name.
- %a
- is replaced by the locale's abbreviated weekday name.
- %B
- is replaced by the locale's full month name.
- %bor- %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 [00,99].
- %c
- is replaced by the locale's appropriate date and time representation.
- %D
- is replaced by the date in the format
      “%m/%d/%y”.
- %d
- is replaced by the day of the month as a decimal number [01,31].
- %e
- is replaced by the day of month as a decimal number [1,31]; single digits
      are preceded by a blank.
- %F
- is equivalent to “%Y-%m-%d” (the ISO
      8601 date format).
- %G
- is replaced by the ISO 8601 year with century as a decimal number. See
      also the %Vconversion specification
- %g
- is replaced by the ISO 8601 year without century as a decimal number
      [00-99]. 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.
- %H
- is replaced by the hour (24-hour clock) as a decimal number [00,23].
- %I
- is replaced by the hour (12-hour clock) as a decimal number [01,12].
- %j
- is replaced by the day of the year as a decimal number [001,366].
- %k
- is replaced by the hour (24-hour clock) as a decimal number [0,23]; single
      digits are preceded by a blank.
- %l
- is replaced by the hour (12-hour clock) as a decimal number [1,12]; single
      digits are preceded by a blank.
- %M
- is replaced by the minute as a decimal number [00,59].
- %m
- is replaced by the month as a decimal number [01,12].
- %n
- is replaced by a newline.
- %p
- is replaced by the locale's equivalent of either “AM” or
      “PM”.
- %R
- is replaced by the time in the format
      “%H:%M”.
- %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.
- %s
- is replaced by the number of seconds since the Epoch (see
      ctime(3)).
- %T
- is replaced by the time in the format
      “%H:%M:%S”.
- %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].
- %u
- is replaced by the weekday (Monday as the first day of the week) as a
      decimal number [1,7].
- %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.
- %v
- is replaced by the date in the format
      “%e-%b-%Y”.
- %W
- is replaced by the week number of the year (Monday as the first day of the
      week) as a decimal number [00,53].
- %w
- is replaced by the weekday (Sunday as the first day of the week) as a
      decimal number [0,6].
- %X
- is replaced by the locale's appropriate time representation.
- %x
- is replaced by the locale's appropriate date representation.
- %Y
- is replaced by the year with century as a decimal number.
- %y
- is replaced by the year without century as a decimal number [00,99].
- %Z
- is replaced by the time zone abbreviation, or the empty string if this is
      not determinable.
- %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.
- %-*
- 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 ‘%’.
The strftime_z() function is similar to
    strftime(), but it also takes a
    const timezone_t tz
  argument.
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.
This function fails if:
  - [ERANGE]
- The specified file offset is invalid. The total number of resulting bytes,
      including the terminating NULcharacter, is more
      than maxsize.
- [EOVERFLOW]
- The format includes an %sconversion and the
      number of seconds since the Epoch cannot be represented in a
      time_t.
Thestrftime() function conforms to
  ISO/IEC 9899:1999 (“ISO C99”).
  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.
There is no conversion specification for the phase of the moon.
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.