strftime

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



NAME
       strftime - format date and time

SYNOPSIS
       #include <time.h>

       size_t strftime(char *restrict s, size_t max,
                       const char *restrict format,
                       const struct tm *restrict 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 character
       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_langinfo(3) with ABDAY_{17} 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_{17} as an argument.)

       %b     The abbreviated month name according to the current locale.
              (Calculated from tm_mon.)  (The specific names used in the current
              locale can be obtained by calling nl_langinfo(3) with ABMON_{112}
              as an argument.)

       %B     The full month name according to the current locale.  (Calculated
              from tm_mon.)  (The specific names used in the current locale can
              be obtained by calling nl_langinfo(3) with MON_{112} as an
              argument.)

       %c     The preferred date and time representation for the current locale.
              (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 ambiguous
              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 decimal
              number.  The 4-digit year corresponding to the ISO week number
              (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.)
              (Calculated 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.)
              (Calculated 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_langinfo(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
              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.  (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
              specification.)  (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
       conversion specifier character by the E or O modifier to indicate that an
       alternative format should be used.  If the alternative format or
       specification 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 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
       (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 format
       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
       conversions given in ANSI C (unmarked), those given in the Single UNIX
       Specification (marked SU), those given in Olson's timezone package
       (marked TZ), and those given in glibc (marked GNU), except that %+ is not
       supported in glibc2.  On the other hand glibc2 has several more
       extensions.  POSIX.1 only refers to ANSI C; POSIX.2 describes under
       date(1) several extensions that could apply to strftime() as well.  The
       %F conversion 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 conversion
       specifier character, an optional flag and field width may be specified.
       (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 programmers
       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 5.11 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                                2021-03-22                        STRFTIME(3)