STRFTIME(3)



STRFTIME(3)                Linux Programmer's Manual               STRFTIME(3)

NAME
       strftime - format date and time

SYNOPSIS
       #include <time.h>

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

DESCRIPTION
       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 broken-down time structure tm is defined in
       <time.h>.  See also ctime(3).

       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 shown in the list below.   In
       this list, the field(s) employed from the tm structure are also shown.

       %a     The  abbreviated  name  of  the day of the week according to the
              current locale.  (Calculated from tm_wday.)

       %A     The full name of the day of the week according  to  the  current
              locale.  (Calculated from tm_wday.)

       %b     The  abbreviated  month  name  according  to the current locale.
              (Calculated from tm_mon.)

       %B     The full month name according to the  current  locale.   (Calcu-
              lated from tm_mon.)

       %c     The  preferred  date  and  time  representation  for the current
              locale.

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

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

       %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) (Calculated from tm_mday.)

       %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) (Calculated from  tm_year,
              tm_yday, and tm_wday.)

       %g     Like  %G,  but  without  century,  that  is, with a 2-digit year
              (00-99). (TZ) (Calculated from tm_year, tm_yday, and tm_wday.)

       %h     Equivalent to %b.  (SU)

       %H     The hour as a decimal number using a 24-hour clock (range 00  to
              23).  (Calculated from tm_hour.)

       %I     The  hour as a decimal number using a 12-hour clock (range 01 to
              12).  (Calculated from tm_hour.)

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

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

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

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

       %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".  (Calculated from tm_hour.)

       %P     Like %p but in lowercase: "am" or "pm" or a corresponding string
              for the current locale.  (Calculated from tm_hour.)  (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) (Calculated from mktime(tm).)

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

       %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.  (Calculated from tm_wday.)  (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.   (Calculated  from  tm_yday  and
              tm_wday.)

       %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.
              (Calculated from tm_year, tm_yday, and tm_wday.)  (SU)

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

       %W     The  week  number of the current year as a decimal number, range
              00 to 53, starting with the first Monday as  the  first  day  of
              week 01.  (Calculated from tm_yday and tm_wday.)

       %x     The preferred date representation for the current locale without
              the time.

       %X     The preferred time representation for the current locale without
              the date.

       %y     The year as a decimal number without a century (range 00 to 99).
              (Calculated from tm_year)

       %Y     The year as a decimal number including the century.  (Calculated
              from tm_year)

       %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
              glibc2.)

       %%     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.

RETURN VALUE
       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.

ENVIRONMENT
       The environment variables TZ and LC_TIME are used.

ATTRIBUTES
       For   an   explanation   of   the  terms  used  in  this  section,  see
       attributes(7).

       +-----------+---------------+--------------------+
       |Interface  | Attribute     | Value              |
       +-----------+---------------+--------------------+
       |strftime() | Thread safety | MT-Safe env locale |
       +-----------+---------------+--------------------+
CONFORMING TO
       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
       the theoretical possibility of a minute that  included  a  double  leap
       second (there never has been such a minute).

NOTES
   ISO 8601 week dates
       %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.

BUGS
       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

           size_t
           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
       warning, so that the above workaround is no longer required.

EXAMPLE
       RFC 2822-compliant date format (with an English locale for %a and %b)

         "%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>

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

           t = time(NULL);
           tmp = localtime(&t);
           if (tmp == NULL) {
               perror("localtime");
               exit(EXIT_FAILURE);
           }

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

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

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

COLOPHON
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       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

GNU                               2016-03-15                       STRFTIME(3)

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