#include <time.h>

       size_t strftime(char *s, size_t max, const char *format,
                       const struct tm *tm);

       The  strftime()  function  formats the broken-down time tm according to
       the format specification format and places the result in the  character
       array s of size max.

       The  format  specification  is a null-terminated string and may contain
       special character sequences called conversion specifications,  each  of
       which  is  introduced  by  a '%' character and terminated by some other
       character known as a conversion specifier character.  All other charac-
       ter sequences are ordinary character sequences.

       The  characters  of  ordinary  character  sequences (including the null
       byte) are copied verbatim from format to s.  However, the characters of
       conversion specifications are replaced as follows:

       %a     The abbreviated weekday name according to the current locale.

       %A     The full weekday name according to the current locale.

       %b     The abbreviated month name according to the current locale.

       %B     The full month name according to the current locale.

       %c     The  preferred  date  and  time  representation  for the current

       %C     The century number (year/100) as a 2-digit integer. (SU)

       %d     The day of the month as a decimal number (range 01 to 31).

       %D     Equivalent to %m/%d/%y.  (Yecch--for Americans only.   Americans
              should  note  that in other countries %d/%m/%y is rather common.
              This means that in international context this format is  ambigu-
              ous and should not be used.) (SU)

       %e     Like %d, the day of the month as a decimal number, but a leading
              zero is replaced by a space. (SU)

       %E     Modifier: use alternative format, see below. (SU)

       %F     Equivalent to %Y-%m-%d (the ISO 8601 date format). (C99)

       %G     The ISO 8601 week-based year (see NOTES) with century as a deci-
              mal number.  The 4-digit year corresponding to the ISO week num-
              ber (see %V).  This has the same format and value as %Y,  except
              that  if  the  ISO  week  number belongs to the previous or next
              year, that year is used instead. (TZ)

       %j     The day of the year as a decimal number (range 001 to 366).

       %k     The  hour  (24-hour  clock) as a decimal number (range 0 to 23);
              single digits are preceded by a blank.  (See also %H.)  (TZ)

       %l     The hour (12-hour clock) as a decimal number (range  1  to  12);
              single digits are preceded by a blank.  (See also %I.)  (TZ)

       %m     The month as a decimal number (range 01 to 12).

       %M     The minute as a decimal number (range 00 to 59).

       %n     A newline character. (SU)

       %O     Modifier: use alternative format, see below. (SU)

       %p     Either  "AM"  or  "PM" according to the given time value, or the
              corresponding strings for the current locale.  Noon  is  treated
              as "PM" and midnight as "AM".

       %P     Like %p but in lowercase: "am" or "pm" or a corresponding string
              for the current locale. (GNU)

       %r     The time in a.m. or p.m. notation.  In the POSIX locale this  is
              equivalent to %I:%M:%S %p.  (SU)

       %R     The  time  in  24-hour  notation  (%H:%M).   (SU)  For a version
              including the seconds, see %T below.

       %s     The number of seconds since the Epoch, 1970-01-01 00:00:00 +0000
              (UTC). (TZ)

       %S     The  second as a decimal number (range 00 to 60).  (The range is
              up to 60 to allow for occasional leap seconds.)

       %t     A tab character. (SU)

       %T     The time in 24-hour notation (%H:%M:%S).  (SU)

       %u     The day of the week as a decimal, range 1 to 7, Monday being  1.
              See also %w.  (SU)

       %U     The  week  number of the current year as a decimal number, range
              00 to 53, starting with the first Sunday as  the  first  day  of
              week 01.  See also %V and %W.

       %V     The  ISO 8601  week  number (see NOTES) of the current year as a
              decimal number, range 01 to 53, where week 1 is the  first  week
              that  has  at least 4 days in the new year.  See also %U and %W.

       %w     The day of the week as a decimal, range 0 to 6, Sunday being  0.
              See also %u.

       %Y     The year as a decimal number including the century.

       %z     The  +hhmm  or  -hhmm  numeric  timezone  (that is, the hour and
              minute offset from UTC). (SU)

       %Z     The timezone name or abbreviation.

       %+     The date and time in date(1)  format.  (TZ)  (Not  supported  in

       %%     A literal '%' character.

       Some conversion specifications can be modified by preceding the conver-
       sion specifier character by the E or O modifier  to  indicate  that  an
       alternative format should be used.  If the alternative format or speci-
       fication does not exist for the current locale, the behavior will be as
       if  the  unmodified conversion specification were used. (SU) The Single
       UNIX Specification mentions %Ec, %EC, %Ex, %EX,  %Ey,  %EY,  %Od,  %Oe,
       %OH, %OI, %Om, %OM, %OS, %Ou, %OU, %OV, %Ow, %OW, %Oy, where the effect
       of the O modifier is to use alternative  numeric  symbols  (say,  roman
       numerals),  and  that  of  the  E modifier is to use a locale-dependent
       alternative representation.

       The broken-down time structure tm is defined  in  <time.h>.   See  also

       Provided  that  the result string, including the terminating null byte,
       does not exceed max bytes,  strftime()  returns  the  number  of  bytes
       (excluding  the  terminating  null byte) placed in the array s.  If the
       length of the result string (including the terminating null byte) would
       exceed  max  bytes,  then strftime() returns 0, and the contents of the
       array are undefined.  (This behavior applies since at least libc 4.4.4;
       very  old versions of libc, such as libc 4.4.1, would return max if the
       array was too small.)

       Note that the return value 0 does not necessarily  indicate  an  error.
       For  example, in many locales %p yields an empty string.  An empty for-
       mat string will likewise yield an empty string.

       The environment variables TZ and LC_TIME are used.

       SVr4, C89, C99.  There are strict inclusions between the set of conver-
       sions given in ANSI C (unmarked), those given in the Single UNIX Speci-
       fication (marked SU), those given in Olson's timezone  package  (marked
       TZ),  and those given in glibc (marked GNU), except that %+ is not sup-
       ported in glibc2.  On the other hand glibc2  has  several  more  exten-
       sions.   POSIX.1 only refers to ANSI C; POSIX.2 describes under date(1)
       several extensions that could apply to strftime() as well.  The %F con-
       version is in C99 and POSIX.1-2001.

       In  SUSv2,  the  %S specifier allowed a range of 00 to 61, to allow for
       within that year, then the ISO 8601 week-based system counts those days
       as  part of week 53 of the preceding year.  For example, 1 January 2010
       is a Friday, meaning that just three days of that calendar week fall in
       2010.   Thus, the ISO 8601 week-based system considers these days to be
       part of week 53 (%V) of the year 2009 (%G); week 01  of  ISO 8601  year
       2010 starts on Monday, 4 January 2010.

   Glibc notes
       Glibc  provides  some extensions for conversion specifications.  (These
       extensions are not specified in POSIX.1-2001, but a few  other  systems
       provide  similar  features.)  Between the '%' character and the conver-
       sion specifier character, an optional flag and field width may be spec-
       ified.  (These precede the E or O modifiers, if present.)

       The following flag characters are permitted:

       _      (underscore) Pad a numeric result string with spaces.

       -      (dash) Do not pad a numeric result string.

       0      Pad  a  numeric  result string with zeros even if the conversion
              specifier character uses space-padding by default.

       ^      Convert alphabetic characters in result string to upper case.

       #      Swap the case of the result string.  (This flag works only  with
              certain  conversion  specifier  characters,  and of these, it is
              only really useful with %Z.)

       An optional decimal width specifier may follow  the  (possibly  absent)
       flag.   If  the  natural  size of the field is smaller than this width,
       then the result string is padded (on the left) to the specified width.

       If the output string would exceed max bytes, errno is  not  set.   This
       makes it impossible to distinguish this error case from cases where the
       format  string  legitimately  produces  a  zero-length  output  string.
       POSIX.1-2001 does not specify any errno settings for strftime().

       Some  buggy  versions  of gcc(1) complain about the use of %c: warning:
       `%c' yields only last 2 digits of year in some locales.  Of course pro-
       grammers are encouraged to use %c, it gives the preferred date and time
       representation.  One meets all kinds of strange obfuscations to circum-
       vent this gcc(1) problem.  A relatively clean one is to add an interme-
       diate function

           my_strftime(char *s, size_t max, const char *fmt,
                       const struct tm *tm)
               return strftime(s, max, fmt, tm);

       Nowadays, gcc(1) provides the -Wno-format-y2k  option  to  prevent  the
       The program below can be used to experiment with strftime().

       Some examples of the result string produced by the glibc implementation
       of strftime() are as follows:

           $ ./a.out '%m'
           Result string is "11"
           $ ./a.out '%5m'
           Result string is "00011"
           $ ./a.out '%_5m'
           Result string is "   11"

       Here's the program source:

       #include <time.h>
       #include <stdio.h>
       #include <stdlib.h>

       main(int argc, char *argv[])
           char outstr[200];
           time_t t;
           struct tm *tmp;

           t = time(NULL);
           tmp = localtime(&t);
           if (tmp == NULL) {

           if (strftime(outstr, sizeof(outstr), argv[1], tmp) == 0) {
               fprintf(stderr, "strftime returned 0");

           printf("Result string is \"%s\"\n", outstr);

       date(1), time(2), ctime(3), setlocale(3), sprintf(3), strptime(3)

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       be found at

GNU                               2013-06-28                       STRFTIME(3)
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