std::strftime(3)

NAME
std::strftime - std::strftime

Synopsis
   Defined in header <ctime>
   std::size_t strftime( char* str, std::size_t count, const char* format, const
   std::tm* tp );
   Converts the date and time information from a given calendar time tp to a
   null-terminated multibyte character string str according to format string format. Up
   to count bytes are written.

Parameters
   str - pointer to the first element of the char array for output
   count - maximum number of bytes to write
   format - pointer to a null-terminated multibyte character string specifying the
            format of conversion
   tp - pointer to the object containing date and time information to be converted
   Format string
   The format string consists of zero or more conversion specifiers and ordinary
   characters (except %). All ordinary characters, including the terminating null
   character, are copied to the output string without modification. Each conversion
   specification begins with % character, optionally followed by E or O modifier
   (ignored if unsupported by the locale), followed by the character that determines
   the behavior of the specifier. The following format specifiers are available:
   Conversion Explanation Used fields
   specifier
       % writes literal %. The full conversion specification
              must be %%.
       n writes newline character
    (C++11)
       t writes horizontal tab character
    (C++11)
                        Year
       Y writes year as a decimal number, e.g. 2017 tm_year
       EY writes year in the alternative representation, e.g.平
    (C++11) 成23年 (year Heisei 23) instead of 2011年 (year 2011) tm_year
              in ja_JP locale
       y writes last 2 digits of year as a decimal number (range tm_year
              [00,99])
       Oy writes last 2 digits of year using the alternative tm_year
    (C++11) numeric system, e.g. 十一 instead of 11 in ja_JP locale
       Ey writes year as offset from locale's alternative tm_year
    (C++11) calendar period %EC (locale-dependent)
       C writes first 2 digits of year as a decimal number tm_year
    (C++11) (range [00,99])
       EC writes name of the base year (period) in the locale's
    (C++11) alternative representation, e.g. 平成 (Heisei era) in tm_year
              ja_JP
              writes ISO 8601 week-based year, i.e. the year that
              contains the specified week.
       G In ISO 8601 weeks begin with Monday and the first week tm_year, tm_wday,
    (C++11) of the year must satisfy the following requirements: tm_yday
                * Includes January 4
                * Includes first Thursday of the year
              writes last 2 digits of ISO 8601 week-based year, i.e.
              the year that contains the specified week (range
              [00,99]).
       g tm_year, tm_wday,
    (C++11) In ISO 8601 weeks begin with Monday and the first week tm_yday
              of the year must satisfy the following requirements:
                * Includes January 4
                * Includes first Thursday of the year
                        Month
       b writes abbreviated month name, e.g. Oct (locale tm_mon
              dependent)
       h synonym of b tm_mon
    (C++11)
       B writes full month name, e.g. October (locale dependent) tm_mon
       m writes month as a decimal number (range [01,12]) tm_mon
       Om writes month using the alternative numeric system, e.g. tm_mon
    (C++11) 十二 instead of 12 in ja_JP locale
                        Week
       U writes week of the year as a decimal number (Sunday is tm_year, tm_wday,
              the first day of the week) (range [00,53]) tm_yday
       OU writes week of the year, as by %U, using the tm_year, tm_wday,
    (C++11) alternative numeric system, e.g. 五十二 instead of 52 tm_yday
              in ja_JP locale
       W writes week of the year as a decimal number (Monday is tm_year, tm_wday,
              the first day of the week) (range [00,53]) tm_yday
       OW writes week of the year, as by %W, using the tm_year, tm_wday,
    (C++11) alternative numeric system, e.g. 五十二 instead of 52 tm_yday
              in ja_JP locale
              writes ISO 8601 week of the year (range [01,53]).
       V In ISO 8601 weeks begin with Monday and the first week tm_year, tm_wday,
    (C++11) of the year must satisfy the following requirements: tm_yday
                * Includes January 4
                * Includes first Thursday of the year
       OV writes week of the year, as by %V, using the tm_year, tm_wday,
    (C++11) alternative numeric system, e.g. 五十二 instead of 52 tm_yday
              in ja_JP locale
                        Day of the year/month
       j writes day of the year as a decimal number (range tm_yday
              [001,366])
       d writes day of the month as a decimal number (range tm_mday
              [01,31])
              writes zero-based day of the month using the
       Od alternative numeric system, e.g. 二十七 instead of 27
    (C++11) in ja_JP locale tm_mday
              Single character is preceded by a space.
              writes day of the month as a decimal number (range
       e [1,31]). tm_mday
    (C++11)
              Single digit is preceded by a space.
              writes one-based day of the month using the alternative
       Oe numeric system, e.g. 二十七 instead of 27 in ja_JP
    (C++11) locale tm_mday
              Single character is preceded by a space.
                        Day of the week
       a writes abbreviated weekday name, e.g. Fri (locale tm_wday
              dependent)
       A writes full weekday name, e.g. Friday (locale tm_wday
              dependent)
       w writes weekday as a decimal number, where Sunday is 0 tm_wday
              (range [0-6])
       Ow writes weekday, where Sunday is 0, using the
    (C++11) alternative numeric system, e.g. 二 instead of 2 in tm_wday
              ja_JP locale
       u writes weekday as a decimal number, where Monday is 1 tm_wday
    (C++11) (ISO 8601 format) (range [1-7])
       Ou writes weekday, where Monday is 1, using the
    (C++11) alternative numeric system, e.g. 二 instead of 2 in tm_wday
              ja_JP locale
                        Hour, minute, second
       H writes hour as a decimal number, 24 hour clock (range tm_hour
              [00-23])
       OH writes hour from 24-hour clock using the alternative tm_hour
    (C++11) numeric system, e.g. 十八 instead of 18 in ja_JP locale
       I writes hour as a decimal number, 12 hour clock (range tm_hour
              [01,12])
       OI writes hour from 12-hour clock using the alternative tm_hour
    (C++11) numeric system, e.g. 六 instead of 06 in ja_JP locale
       M writes minute as a decimal number (range [00,59]) tm_min
       OM writes minute using the alternative numeric system, tm_min
    (C++11) e.g. 二十五 instead of 25 in ja_JP locale
       S writes second as a decimal number (range [00,60]) tm_sec
       OS writes second using the alternative numeric system, tm_sec
    (C++11) e.g. 二十四 instead of 24 in ja_JP locale

Other
       c writes standard date and time string, e.g. Sun Oct 17 all
              04:41:13 2010 (locale dependent)
       Ec writes alternative date and time string, e.g. using 平
    (C++11) 成23年 (year Heisei 23) instead of 2011年 (year 2011) all
              in ja_JP locale
       x writes localized date representation (locale dependent) all
       Ex writes alternative date representation, e.g. using 平
    (C++11) 成23年 (year Heisei 23) instead of 2011年 (year 2011) all
              in ja_JP locale
       X writes localized time representation, e.g. 18:40:20 or all
              6:40:20 PM (locale dependent)
       EX writes alternative time representation (locale all
    (C++11) dependent)
       D equivalent to "%m/%d/%y" tm_mon, tm_mday,
    (C++11) tm_year
       F equivalent to "%Y-%m-%d" (the ISO 8601 date format) tm_mon, tm_mday,
    (C++11) tm_year
       r writes localized 12-hour clock time (locale dependent) tm_hour, tm_min,
    (C++11) tm_sec
       R equivalent to "%H:%M" tm_hour, tm_min
    (C++11)
       T equivalent to "%H:%M:%S" (the ISO 8601 time format) tm_hour, tm_min,
    (C++11) tm_sec
       p writes localized a.m. or p.m. (locale dependent) tm_hour
       z writes offset from UTC in the ISO 8601 format (e.g.
    (C++11) -0430), or no characters if the time zone information tm_isdst
              is not available
              writes locale-dependent time zone name or abbreviation,
       Z or no characters if the time zone information is not tm_isdst
              available

Return value
   The number of bytes written into the character array pointed to by str not including
   the terminating '\0' on success. If count was reached before the entire string could
   be stored, 0 is returned and the contents are indeterminate.

Example
// Run this code
 #include <ctime>
 #include <iostream>
 #include <iterator>
 #include <locale>
 void utcExample()
 {
     // Example of the very popular RFC 3339 format UTC time
     std::time_t time = std::time({});
     char timeString[std::size("yyyy-mm-ddThh:mm:ssZ")];
     std::strftime(std::data(timeString), std::size(timeString),
                   "%FT%TZ", std::gmtime(&time));
     std::cout << timeString << '\n';
 }
 int main()
 {
     std::time_t t = std::time(nullptr);
     char mbstr[100];
     if (std::strftime(mbstr, sizeof(mbstr), "%A %c", std::localtime(&t)))
         std::cout << mbstr << '\n';
     std::locale::global(std::locale("ja_JP.utf8"));
     if (std::strftime(mbstr, sizeof(mbstr), "%A %c", std::localtime(&t)))
         std::cout << mbstr << '\n';
     utcExample();
 }

Possible output:
 Tuesday Tue Sep 7 19:40:35 2021
 火曜日 2021年09月07日 19時40分35秒
 2021-09-07T19:40:35Z

See also
                        converts a std::tm object to a textual
   asctime representation
                        (function)
                        converts a std::time_t object to a textual
   ctime representation
                        (function)
                        converts a std::tm object to custom wide
   wcsftime string textual representation
                        (function)
   put_time formats and outputs a date/time value
   (C++11) according to the specified format
                        (function template)
   std::formatter<std::chrono::hh_mm_ss> formatting support for hh_mm_ss
   (C++20) (class template specialization)
   C documentation for
   strftime