strftime(3) - format date and time



  • 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
           This page is part of release 4.15 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				  2017-09-15			   STRFTIME(3)
    

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