#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  name  of  the day of the week according to the
              current locale.

       %A     The full name of the day of the week according  to  the  current

       %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.

       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.

       For  an  explanation  of  the  terms  used   in   this   section,   see

       |Interface  | Attribute     | Value              |
       |strftime() | Thread safety | MT-Safe env locale |
       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
       %G, %g, and %V yield values calculated from the week-based year defined
       by the ISO 8601 standard.  In this system, weeks start on a Monday, and
       are numbered from 01, for the first week, up to 52 or 53, for the  last
       week.  Week 1 is the first week where four or more days fall within the
       new year (or, synonymously, week 01 is: the first week of the year that
       contains  a  Thursday;  or,  the  week that has 4 January in it).  When
       three of fewer days of the first calendar week of  the  new  year  fall
       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 uppercase.

       #      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


         "%a, %d %b %Y %T %z"

       RFC 822-compliant date format (with an English locale for %a and %b)

         "%a, %d %b %y %T %z"

   Example program
       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"

   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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