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STRFTIME(3P)              POSIX Programmer's Manual             STRFTIME(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.

NAME         top

       strftime, strftime_l — convert date and time to a string

SYNOPSIS         top

       #include <time.h>

       size_t strftime(char *restrict s, size_t maxsize,
           const char *restrict format, const struct tm *restrict timeptr);
       size_t strftime_l(char *restrict s, size_t maxsize,
           const char *restrict format, const struct tm *restrict timeptr,
           locale_t locale);

DESCRIPTION         top

       For strftime(): The functionality described on this reference page is
       aligned with the ISO C standard. Any conflict between the
       requirements described here and the ISO C standard is unintentional.
       This volume of POSIX.1‐2008 defers to the ISO C standard.

       The strftime() function shall place bytes into the array pointed to
       by s as controlled by the string pointed to by format.  The format is
       a character string, beginning and ending in its initial shift state,
       if any. The format string consists of zero or more conversion
       specifications and ordinary characters.

       Each conversion specification is introduced by the '%' character
       after which the following appear in sequence:

        *  An optional flag:

           0     The zero character ('0'), which specifies that the
                 character used as the padding character is '0',

           +     The <plus-sign> character ('+'), which specifies that the
                 character used as the padding character is '0', and that if
                 and only if the field being produced consumes more than
                 four bytes to represent a year (for %F, %G, or %Y) or more
                 than two bytes to represent the year divided by 100 (for
                 %C) then a leading <plus-sign> character shall be included
                 if the year being processed is greater than or equal to
                 zero or a leading minus-sign character ('−') shall be
                 included if the year is less than zero.

           The default padding character is unspecified.

        *  An optional minimum field width. If the converted value,
           including any leading '+' or '−' sign, has fewer bytes than the
           minimum field width and the padding character is not the NUL
           character, the output shall be padded on the left (after any
           leading '+' or '−' sign) with the padding character.

        *  An optional E or O modifier.

        *  A terminating conversion specifier character that indicates the
           type of conversion to be applied.

       The results are unspecified if more than one flag character is
       specified, a flag character is specified without a minimum field
       width; a minimum field width is specified without a flag character; a
       modifier is specified with a flag or with a minimum field width; or
       if a minimum field width is specified for any conversion specifier
       other than C, F, G, or Y.

       All ordinary characters (including the terminating NUL character) are
       copied unchanged into the array. If copying takes place between
       objects that overlap, the behavior is undefined. No more than maxsize
       bytes are placed into the array. Each conversion specifier is
       replaced by appropriate characters as described in the following
       list. The appropriate characters are determined using the LC_TIME
       category of the current locale and by the values of zero or more
       members of the broken-down time structure pointed to by timeptr, as
       specified in brackets in the description. If any of the specified
       values are outside the normal range, the characters stored are
       unspecified.

       The strftime_l() function shall be equivalent to the strftime()
       function, except that the locale data used is from the locale
       represented by locale.

       Local timezone information is used as though strftime() called
       tzset().

       The following conversion specifiers shall be supported:

       a       Replaced by the locale's abbreviated weekday name. [tm_wday]

       A       Replaced by the locale's full weekday name. [tm_wday]

       b       Replaced by the locale's abbreviated month name. [tm_mon]

       B       Replaced by the locale's full month name. [tm_mon]

       c       Replaced by the locale's appropriate date and time
               representation.  (See the Base Definitions volume of
               POSIX.1‐2008, <time.h>.)

       C       Replaced by the year divided by 100 and truncated to an
               integer, as a decimal number. [tm_year]

               If a minimum field width is not specified, the number of
               characters placed into the array pointed to by s will be the
               number of digits in the year divided by 100 or two, whichever
               is greater.  If a minimum field width is specified, the
               number of characters placed into the array pointed to by s
               will be the number of digits in the year divided by 100 or
               the minimum field width, whichever is greater.

       d       Replaced by the day of the month as a decimal number [01,31].
               [tm_mday]

       D       Equivalent to %m/%d/%y.  [tm_mon, tm_mday, tm_year]

       e       Replaced by the day of the month as a decimal number [1,31];
               a single digit is preceded by a space. [tm_mday]

       F       Equivalent to %+4Y-%m-%d if no flag and no minimum field
               width are specified. [tm_year, tm_mon, tm_mday]

               If a minimum field width of x is specified, the year shall be
               output as if by the Y specifier (described below) with
               whatever flag was given and a minimum field width of x−6.  If
               x is less than 6, the behavior shall be as if x equalled 6.

               If the minimum field width is specified to be 10, and the
               year is four digits long, then the output string produced
               will match the ISO 8601:2004 standard subclause 4.1.2.2
               complete representation, extended format date representation
               of a specific day. If a + flag is specified, a minimum field
               width of x is specified, and x−7 bytes are sufficient to hold
               the digits of the year (not including any needed sign
               character), then the output will match the ISO 8601:2004
               standard subclause 4.1.2.4 complete representation, expanded
               format date representation of a specific day.

       g       Replaced by the last 2 digits of the week-based year (see
               below) as a decimal number [00,99]. [tm_year, tm_wday,
               tm_yday]

       G       Replaced by the week-based year (see below) as a decimal
               number (for example, 1977). [tm_year, tm_wday, tm_yday]

               If a minimum field width is specified, the number of
               characters placed into the array pointed to by s will be the
               number of digits and leading sign characters (if any) in the
               year, or the minimum field width, whichever is greater.

       h       Equivalent to %b.  [tm_mon]

       H       Replaced by the hour (24-hour clock) as a decimal number
               [00,23]. [tm_hour]

       I       Replaced by the hour (12-hour clock) as a decimal number
               [01,12]. [tm_hour]

       j       Replaced by the day of the year as a decimal number
               [001,366]. [tm_yday]

       m       Replaced by the month as a decimal number [01,12]. [tm_mon]

       M       Replaced by the minute as a decimal number [00,59]. [tm_min]

       n       Replaced by a <newline>.

       p       Replaced by the locale's equivalent of either a.m. or p.m.
               [tm_hour]

       r       Replaced by the time in a.m. and p.m. notation; in the POSIX
               locale this shall be equivalent to %I:%M:%S %p.  [tm_hour,
               tm_min, tm_sec]

       R       Replaced by the time in 24-hour notation (%H:%M).  [tm_hour,
               tm_min]

       S       Replaced by the second as a decimal number [00,60]. [tm_sec]

       t       Replaced by a <tab>.

       T       Replaced by the time (%H:%M:%S).  [tm_hour, tm_min, tm_sec]

       u       Replaced by the weekday as a decimal number [1,7], with 1
               representing Monday. [tm_wday]

       U       Replaced by the week number of the year as a decimal number
               [00,53].  The first Sunday of January is the first day of
               week 1; days in the new year before this are in week 0.
               [tm_year, tm_wday, tm_yday]

       V       Replaced by the week number of the year (Monday as the first
               day of the week) as a decimal number [01,53]. If the week
               containing 1 January has four or more days in the new year,
               then it is considered week 1.  Otherwise, it is the last week
               of the previous year, and the next week is week 1. Both
               January 4th and the first Thursday of January are always in
               week 1. [tm_year, tm_wday, tm_yday]

       w       Replaced by the weekday as a decimal number [0,6], with 0
               representing Sunday. [tm_wday]

       W       Replaced by the week number of the year as a decimal number
               [00,53].  The first Monday of January is the first day of
               week 1; days in the new year before this are in week 0.
               [tm_year, tm_wday, tm_yday]

       x       Replaced by the locale's appropriate date representation.
               (See the Base Definitions volume of POSIX.1‐2008, <time.h>.)

       X       Replaced by the locale's appropriate time representation.
               (See the Base Definitions volume of POSIX.1‐2008, <time.h>.)

       y       Replaced by the last two digits of the year as a decimal
               number [00,99]. [tm_year]

       Y       Replaced by the year as a decimal number (for example, 1997).
               [tm_year]

               If a minimum field width is specified, the number of
               characters placed into the array pointed to by s will be the
               number of digits and leading sign characters (if any) in the
               year, or the minimum field width, whichever is greater.

       z       Replaced by the offset from UTC in the ISO 8601:2004 standard
               format (+hhmm or −hhmm), or by no characters if no timezone
               is determinable. For example, "−0430" means 4 hours 30
               minutes behind UTC (west of Greenwich).  If tm_isdst is zero,
               the standard time offset is used. If tm_isdst is greater than
               zero, the daylight savings time offset is used. If tm_isdst
               is negative, no characters are returned.  [tm_isdst]

       Z       Replaced by the timezone name or abbreviation, or by no bytes
               if no timezone information exists. [tm_isdst]

       %       Replaced by %.

       If a conversion specification does not correspond to any of the
       above, the behavior is undefined.

       If a struct tm broken-down time structure is created by localtime()
       or localtime_r(), or modified by mktime(), and the value of TZ is
       subsequently modified, the results of the %Z and %z strftime()
       conversion specifiers are undefined, when strftime() is called with
       such a broken-down time structure.

       If a struct tm broken-down time structure is created or modified by
       gmtime() or gmtime_r(), it is unspecified whether the result of the
       %Z and %z conversion specifiers shall refer to UTC or the current
       local timezone, when strftime() is called with such a broken-down
       time structure.

   Modified Conversion Specifiers
       Some conversion specifiers can be modified by the E or O modifier
       characters to indicate that an alternative format or specification
       should be used rather than the one normally used by the unmodified
       conversion specifier. If the alternative format or specification does
       not exist for the current locale (see ERA in the Base Definitions
       volume of POSIX.1‐2008, Section 7.3.5, LC_TIME), the behavior shall
       be as if the unmodified conversion specification were used.

       %Ec     Replaced by the locale's alternative appropriate date and
               time representation.

       %EC     Replaced by the name of the base year (period) in the
               locale's alternative representation.

       %Ex     Replaced by the locale's alternative date representation.

       %EX     Replaced by the locale's alternative time representation.

       %Ey     Replaced by the offset from %EC (year only) in the locale's
               alternative representation.

       %EY     Replaced by the full alternative year representation.

       %Od     Replaced by the day of the month, using the locale's
               alternative numeric symbols, filled as needed with leading
               zeros if there is any alternative symbol for zero; otherwise,
               with leading <space> characters.

       %Oe     Replaced by the day of the month, using the locale's
               alternative numeric symbols, filled as needed with leading
               <space> characters.

       %OH     Replaced by the hour (24-hour clock) using the locale's
               alternative numeric symbols.

       %OI     Replaced by the hour (12-hour clock) using the locale's
               alternative numeric symbols.

       %Om     Replaced by the month using the locale's alternative numeric
               symbols.

       %OM     Replaced by the minutes using the locale's alternative
               numeric symbols.

       %OS     Replaced by the seconds using the locale's alternative
               numeric symbols.

       %Ou     Replaced by the weekday as a number in the locale's
               alternative representation (Monday=1).

       %OU     Replaced by the week number of the year (Sunday as the first
               day of the week, rules corresponding to %U) using the
               locale's alternative numeric symbols.

       %OV     Replaced by the week number of the year (Monday as the first
               day of the week, rules corresponding to %V) using the
               locale's alternative numeric symbols.

       %Ow     Replaced by the number of the weekday (Sunday=0) using the
               locale's alternative numeric symbols.

       %OW     Replaced by the week number of the year (Monday as the first
               day of the week) using the locale's alternative numeric
               symbols.

       %Oy     Replaced by the year (offset from %C) using the locale's
               alternative numeric symbols.

       %g, %G, and %V give values according to the ISO 8601:2004 standard
       week-based year. In this system, weeks begin on a Monday and week 1
       of the year is the week that includes January 4th, which is also the
       week that includes the first Thursday of the year, and is also the
       first week that contains at least four days in the year. If the first
       Monday of January is the 2nd, 3rd, or 4th, the preceding days are
       part of the last week of the preceding year; thus, for Saturday 2nd
       January 1999, %G is replaced by 1998 and %V is replaced by 53. If
       December 29th, 30th, or 31st is a Monday, it and any following days
       are part of week 1 of the following year. Thus, for Tuesday 30th
       December 1997, %G is replaced by 1998 and %V is replaced by 01.

       If a conversion specifier is not one of the above, the behavior is
       undefined.

       The behavior is undefined if the locale argument to strftime_l() is
       the special locale object LC_GLOBAL_LOCALE or is not a valid locale
       object handle.

RETURN VALUE         top

       If the total number of resulting bytes including the terminating null
       byte is not more than maxsize, these functions shall return the
       number of bytes placed into the array pointed to by s, not including
       the terminating NUL character. Otherwise, 0 shall be returned and the
       contents of the array are unspecified.

ERRORS         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top

   Getting a Localized Date String
       The following example first sets the locale to the user's default.
       The locale information will be used in the nl_langinfo() and
       strftime() functions. The nl_langinfo() function returns the
       localized date string which specifies how the date is laid out. The
       strftime() function takes this information and, using the tm
       structure for values, places the date and time information into
       datestring.

           #include <time.h>
           #include <locale.h>
           #include <langinfo.h>
           ...
           struct tm *tm;
           char datestring[256];
           ...
           setlocale (LC_ALL, "");
           ...
           strftime (datestring, sizeof(datestring), nl_langinfo (D_T_FMT), tm);
           ...

APPLICATION USAGE         top

       The range of values for %S is [00,60] rather than [00,59] to allow
       for the occasional leap second.

       Some of the conversion specifications are duplicates of others. They
       are included for compatibility with nl_cxtime() and nl_ascxtime(),
       which were published in Issue 2.

       The %C, %F, %G, and %Y format specifiers in strftime() always print
       full values, but the strptime() %C, %F, and %Y format specifiers only
       scan two digits (assumed to be the first two digits of a four-digit
       year) for %C and four digits (assumed to be the entire (four-digit)
       year) for %F and %Y.  This mimics the behavior of printf() and
       scanf(); that is:

           printf("%2d", x = 1000);

       prints "1000", but:

           scanf(%2d", &x);

       when given "1000" as input will only store 10 in x).  Applications
       using extended ranges of years must be sure that the number of digits
       specified for scanning years with strptime() matches the number of
       digits that will actually be present in the input stream. Historic
       implementations of the %Y conversion specification (with no flags and
       no minimum field width) produced different output formats. Some
       always produced at least four digits (with 0 fill for years from 0
       through 999) while others only produced the number of digits present
       in the year (with no fill and no padding). These two forms can be
       produced with the '0' flag and a minimum field width options using
       the conversions specifications %04Y and %01Y, respectively.

       In the past, the C and POSIX standards specified that %F produced an
       ISO 8601:2004 standard date format, but didn't specify which one. For
       years in the range [0001,9999], POSIX.1‐2008 requires that the output
       produced match the ISO 8601:2004 standard complete representation
       extended format (YYYY-MM-DD) and for years outside of this range
       produce output that matches the ISO 8601:2004 standard expanded
       representation extended format (<+/-><Underline>Y</Underline>YYYY-MM-
       DD). To fully meet ISO 8601:2004 standard requirements, the producer
       and consumer must agree on a date format that has a specific number
       of bytes reserved to hold the characters used to represent the years
       that is sufficiently large to hold all values that will be shared.
       For example, the %+13F conversion specification will produce output
       matching the format "<+/->YYYYYY-MM-DD" (a leading '+' or '−' sign; a
       six-digit, 0-filled year; a '−'; a two-digit, leading 0-filled month;
       another '−'; and the two-digit, leading 0-filled day within the
       month).

       Note that if the year being printed is greater than 9999, the
       resulting string from the unadorned %F conversion specifications will
       not conform to the ISO 8601:2004 standard extended format, complete
       representation for a date and will instead be an extended format,
       expanded representation (presumably without the required agreement
       between the date's producer and consumer).

       In the C locale, the E and O modifiers are ignored and the
       replacement strings for the following specifiers are:

       %a      The first three characters of %A.

       %A      One of Sunday, Monday, ..., Saturday.

       %b      The first three characters of %B.

       %B      One of January, February, ..., December.

       %c      Equivalent to %a %b %e %T %Y.

       %p      One of AM or PM.

       %r      Equivalent to %I:%M:%S %p.

       %x      Equivalent to %m/%d/%y.

       %X      Equivalent to %T.

       %Z      Implementation-defined.

RATIONALE         top

       The %Y conversion specification to strftime() was frequently assumed
       to be a four-digit year, but the ISO C standard does not specify that
       %Y is restricted to any subset of allowed values from the tm_year
       field. Similarly, the %C conversion specification was assumed to be a
       two-digit field and the first part of the output from the %F
       conversion specification was assumed to be a four-digit field. With
       tm_year being a signed 32 or more-bit int and with many current
       implementations supporting 64-bit time_t types in one or more
       programming environments, these assumptions are clearly wrong.

       POSIX.1‐2008 now allows the format specifications %0xC, %0xF, %0xG,
       and %0xY (where 'x' is a string of decimal digits used to specify
       printing and scanning of a string of x decimal digits) with leading
       zero fill characters. Allowing applications to set the field width
       enables them to agree on the number of digits to be printed and
       scanned in the ISO 8601:2004 standard expanded representation of a
       year (for %F, %G, and %Y) or all but the last two digits of the year
       (for %C).  This is based on a feature in some versions of GNU libc's
       strftime().  The GNU version allows specifying space, zero, or no-
       fill characters in strftime() format strings, but does not allow any
       flags to be specified in strptime() format strings. These
       implementations also allow these flags to be specified for any
       numeric field. POSIX.1‐2008 only requires the zero fill flag ('0')
       and only requires that it be recognized when processing %C, %F, %G,
       and %Y specifications when a minimum field width is also specified.
       The '0' flag is the only flag needed to produce and scan the
       ISO 8601:2004 standard year fields using the extended format forms.
       POSIX.1‐2008 also allows applications to specify the same flag and
       field width specifiers to be used in both strftime() and strptime()
       format strings for symmetry. Systems may provide other flag
       characters and may accept flags in conjunction with conversion
       specifiers other than %C, %F, %G, and %Y; but portable applications
       cannot depend on such extensions.

       POSIX.1‐2008 now also allows the format specifications %+xC, %+xF,
       %+xG, and %+xY (where 'x' is a string of decimal digits used to
       specify printing and scanning of a string of 'x' decimal digits) with
       leading zero fill characters and a leading '+' sign character if the
       year being converted is more than four digits or a minimum field
       width is specified that allows room for more than four digits for the
       year. This allows date providers and consumers to agree on a specific
       number of digits to represent a year as required by the ISO 8601:2004
       standard expanded representation formats. The expanded representation
       formats all require the year to begin with a leading '+' or '−' sign.
       (All of these specifiers can also provide a leading '−' sign for
       negative years. Since negative years and the year 0 don't fit well
       with the Gregorian or Julian calendars, the normal ranges of dates
       start with year 1. The ISO C standard allows tm_year to assume values
       corresponding to years before year 1, but the use of such years
       provided unspecified results.)

       Some earlier version of this standard specified that applications
       wanting to use strptime() to scan dates and times printed by
       strftime() should provide non-digit characters between fields to
       separate years from months and days. It also supported %F to print
       and scan the ISO 8601:2004 standard extended format, complete
       representation date for years 1 through 9999 (i.e., YYYY-MM-DD).
       However, many applications were written to print (using strftime())
       and scan (using strptime()) dates written using the basic format
       complete representation (four-digit years) and truncated
       representation (two-digit years) specified by the ISO 8601:2004
       standard representation of dates and times which do not have any
       separation characters between fields. The ISO 8601:2004 standard also
       specifies basic format expanded representation where the creator and
       consumer of these fields agree beforehand to represent years as
       leading zero-filled strings of an agreed length of more than four
       digits to represent a year (again with no separation characters when
       year, month, and day are all displayed). Applications producing and
       consuming expanded representations are encouraged to use the '+' flag
       and an appropriate maximum field width to scan the year including the
       leading sign. Note that even without the '+' flag, years less than
       zero may be represented with a leading minus-sign for %F, %G,and %Y
       conversion specifications. Using negative years results in
       unspecified behavior.

       If a format specification %+xF with the field width x greater than 11
       is specified and the width is large enough to display the full year,
       the output string produced will match the ISO 8601:2004 standard
       subclause 4.1.2.4 expanded representation, extended format date
       representation for a specific day. (For years in the range [1,99999],
       %+12F is sufficient for an agreed five-digit year with a leading sign
       using the ISO 8601:2004 standard expanded representation, extended
       format for a specific day "<+/->YYYYY-MM-DD".)  Note also that years
       less than 0 may produce a leading minus-sign ('−') when using %Y or
       %C whether or not the '0' or '+' flags are used.

       The difference between the '0' flag and the '+' flag is whether the
       leading '+' character will be provided for years >9999 as required
       for the ISO 8601:2004 standard extended representation format
       containing a year. For example:

          ┌───────┬──────────────────────────┬─────────────┬────────────┐
          │       │                          │ strftime()  strptime() │
          │ Year  Conversion Specification Output    Scan Back  │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │1970   │ %Y                       │ 1970        │ 1970       │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │1970   │ %+4Y                     │ 1970        │ 1970       │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │27     │ %Y                       │ 27 or 0027  │ 27         │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │270    │ %Y                       │ 270 or 0270 │ 270        │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │270    │ %+4Y                     │ 0270        │ 270        │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │17     │ %C%y                     │ 0017        │ 17         │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │270    │ %C%y                     │ 0270        │ 270        │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │12345  │ %Y                       │ 12345       │ 1234*      │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │12345  │ %+4Y                     │ +12345      │ 123*       │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │12345  │ %05Y                     │ 12345       │ 12345      │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │270    │ %+5Y or %+3C%y           │ +0270       │ 270        │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │12345  │ %+5Y or %+3C%y           │ +12345      │ 1234*      │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │12345  │ %06Y or %04C%y           │ 012345      │ 12345      │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │12345  │ %+6Y or %+4C%y           │ +12345      │ 12345      │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │123456 │ %08Y or %06C%y           │ 00123456    │ 123456     │
          ├───────┼──────────────────────────┼─────────────┼────────────┤
          │123456 │ %+8Y or %+6C%y           │ +0123456    │ 123456     │
          └───────┴──────────────────────────┴─────────────┴────────────┘
       In the cases above marked with a * in the strptime() scan back field,
       the implied or specified number of characters scanned by strptime()
       was less than the number of characters output by strftime() using the
       same format; so the remaining digits of the year were dropped when
       the output date produced by strftime() was scanned back in by
       strptime().

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       asctime(3p), clock(3p), ctime(3p), difftime(3p), getdate(3p),
       gmtime(3p), localtime(3p), mktime(3p), strptime(3p), time(3p),
       tzset(3p), uselocale(3p), utime(3p)

       The Base Definitions volume of POSIX.1‐2008, Section 7.3.5, LC_TIME,
       time.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) In the event of any discrepancy between this version and
       the original IEEE and The Open Group Standard, the original IEEE and
       The Open Group Standard is the referee document. The original
       Standard can be obtained online at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the
       source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                        STRFTIME(3P)