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.)  (The specific names
              used  in  the current locale can be obtained by calling nl_lang-
              info(3) with ABDAY_{1-7} as an argument.)

       %A     The full name of the day of the week according  to  the  current
              locale.  (Calculated from tm_wday.)  (The specific names used in
              the current locale can be  obtained  by  calling  nl_langinfo(3)
              with DAY_{1-7} as an argument.)

       %b     The  abbreviated  month  name  according  to the current locale.
              (Calculated from tm_mon.)  (The specific names used in the  cur-
              rent  locale  can be obtained by calling nl_langinfo(3) with AB-
              MON_{1-12} as an argument.)

       %B     The full month name according to the  current  locale.   (Calcu-
              lated from tm_mon.)  (The specific names used in the current lo-
              cale can be obtained by calling nl_langinfo(3)  with  MON_{1-12}
              as an argument.)

       %c     The  preferred  date and time representation for the current lo-
              cale.  (The specific format used in the current  locale  can  be
              obtained  by  calling nl_langinfo(3) with D_T_FMT as an argument
              for the %c conversion specification, and  with  ERA_D_T_FMT  for
              the %Ec conversion specification.)  (In the POSIX locale this is
              equivalent to %a %b %e %H:%M:%S %Y.)

       %C     The century number (year/100) as a 2-digit  integer.  (SU)  (The
              %EC  conversion  specification  corresponds  to  the name of the
              era.)  (Calculated 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 ("era-based") 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 numeric symbols, 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.)   (The
              specific  string  representations  used for "AM" and "PM" in the
              current locale can be obtained by  calling  nl_langinfo(3)  with
              AM_STR and PM_STR, respectively.)

       %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.  (SU)  (The  specific  format
              used  in  the current locale can be obtained by calling nl_lang-
              info(3) with T_FMT_AMPM as an argument.)  (In the  POSIX  locale
              this is equivalent to %I:%M:%S %p.)

       %R     The  time  in  24-hour notation (%H:%M).  (SU) For a version in-
              cluding 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.  (The specific format used in the current  locale  can
              be  obtained by calling nl_langinfo(3) with D_FMT as an argument
              for the %x conversion specification, and with ERA_D_FMT for  the
              %Ex  conversion  specification.)   (In  the POSIX locale this is
              equivalent to %m/%d/%y.)

       %X     The preferred time representation for the current locale without
              the  date.   (The specific format used in the current locale can
              be obtained by calling nl_langinfo(3) with T_FMT as an  argument
              for  the %X conversion specification, and with ERA_T_FMT for the
              %EX conversion specification.)  (In the  POSIX  locale  this  is
              equivalent to %H:%M:%S.)

       %y     The year as a decimal number without a century (range 00 to 99).
              (The %Ey conversion specification corresponds to the year  since
              the  beginning of the era denoted by the %EC conversion specifi-
              cation.)  (Calculated from tm_year)

       %Y     The year as a decimal number including the  century.   (The  %EY
              conversion  specification  corresponds  to  the full alternative
              year representation.)  (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 al-
       ternative format should be used.  If the alternative format or specifi-
       cation  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 nu-
       merals), and that of the E modifier is to use a locale-dependent alter-
       native  representation.   The  rules governing date representation with
       the E modifier can be obtained by supplying ERA as  an  argument  to  a
       nl_langinfo(3).   One example of such alternative forms is the Japanese
       era calendar scheme in the ja_JP glibc locale.

RETURN VALUE
       Provided that the result string, including the terminating  null  byte,
       does  not exceed max bytes, strftime() returns the number of bytes (ex-
       cluding 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 at-
       tributes(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  or  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 52 or 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.  Similarly, the
       first two days of January 2011 are considered to be part of week 52  of
       the year 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, as it gives the preferred date and
       time representation.  One meets all kinds of  strange  obfuscations  to
       circumvent  this  gcc(1)  problem.  A relatively clean one is to add an
       intermediate 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.

EXAMPLES
       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), nl_langinfo(3),  setlocale(3),  sprintf(3),
       strptime(3)

COLOPHON
       This  page  is  part of release 5.07 of the Linux man-pages project.  A
       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                               2020-04-11                       STRFTIME(3)

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