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### 8.8 `strftime`—convert date and time to a formatted string

Synopsis

         #include <time.h>
         size_t strftime(char *s, size_t maxsize,
             const char *format, const struct tm *timp);

Description
`strftime` converts a `struct tm` representation of the time (at timp) into a null-terminated string, starting at s and occupying no more than maxsize characters.

You control the format of the output using the string at format. `*`format can contain two kinds of specifications: text to be copied literally into the formatted string, and time conversion specifications. Time conversion specifications are two- and three-character sequences beginning with ``%`' (use ``%%`' to include a percent sign in the output). Each defined conversion specification selects only the specified field(s) of calendar time data from `*`timp, and converts it to a string in one of the following ways:

`%a`
  A three-letter abbreviation for the day of the week. [tm_wday]
`%A`
  The full name for the day of the week, one of ``Sunday`', ``Monday`', ``Tuesday`', ``Wednesday`', ``Thursday`', ``Friday`', or ``Saturday`'. [tm_wday]
`%b`
  A three-letter abbreviation for the month name. [tm_mon]
`%B`
  The full name of the month, one of ``January`', ``February`', ``March`', ``April`', ``May`', ``June`', ``July`', ``August`', ``September`', ``October`', ``November`', ``December`'. [tm_mon]
`%c`
  A string representing the complete date and time, in the form ``"%a %b %e %H:%M:%S %Y"`' (example "Mon Apr 01 13:13:13 1992"). [tm_sec, tm_min, tm_hour, tm_mday, tm_mon, tm_year, tm_wday]
`%C`
  The century, that is, the year divided by 100 then truncated. For 4-digit years, the result is zero-padded and exactly two characters; but for other years, there may a negative sign or more digits. In this way, ``%C%y`' is equivalent to ``%Y`'. [tm_year]
`%d`
  The day of the month, formatted with two digits (from ``01`' to ``31`'). [tm_mday]
`%D`
  A string representing the date, in the form ``"%m/%d/%y"`'. [tm_mday, tm_mon, tm_year]
`%e`
  The day of the month, formatted with leading space if single digit (from ``1`' to ``31`'). [tm_mday]
`%Ex`
  In some locales, the E modifier selects alternative representations of certain modifiers `x`. But in the "C" locale supported by newlib, it is ignored, and treated as %`x`.
`%F`
  A string representing the ISO 8601:2000 date format, in the form ``"%Y-%m-%d"`'. [tm_mday, tm_mon, tm_year]
`%g`
  The last two digits of the week-based year, see specifier %G (from ``00`' to ``99`'). [tm_year, tm_wday, tm_yday]
`%G`
  The week-based year. In the ISO 8601:2000 calendar, week 1 of the year includes January 4th, and begin on Mondays. Therefore, if January 1st, 2nd, or 3rd falls on a Sunday, that day and earlier belong to the last week of the previous year; and if December 29th, 30th, or 31st falls on Monday, that day and later belong to week 1 of the next year. For consistency with %Y, it always has at least four characters. Example: "%G" for Saturday 2nd January 1999 gives "1998", and for Tuesday 30th December 1997 gives "1998". [tm_year, tm_wday, tm_yday]
`%h`
  A three-letter abbreviation for the month name (synonym for "%b"). [tm_mon]
`%H`
  The hour (on a 24-hour clock), formatted with two digits (from ``00`' to ``23`'). [tm_hour]
`%I`
  The hour (on a 12-hour clock), formatted with two digits (from ``01`' to ``12`'). [tm_hour]
`%j`
  The count of days in the year, formatted with three digits (from ``001`' to ``366`'). [tm_yday]
`%k`
  The hour (on a 24-hour clock), formatted with leading space if single digit (from ``0`' to ``23`'). Non-POSIX extension (c.p. %I). [tm_hour]
`%l`
  The hour (on a 12-hour clock), formatted with leading space if single digit (from ``1`' to ``12`'). Non-POSIX extension (c.p. %H). [tm_hour]
`%m`
  The month number, formatted with two digits (from ``01`' to ``12`'). [tm_mon]
`%M`
  The minute, formatted with two digits (from ``00`' to ``59`'). [tm_min]
`%n`
  A newline character (``\n`').
`%Ox`
  In some locales, the O modifier selects alternative digit characters for certain modifiers `x`. But in the "C" locale supported by newlib, it is ignored, and treated as %`x`.
`%p`
  Either ``AM`' or ``PM`' as appropriate. [tm_hour]
`%r`
  The 12-hour time, to the second. Equivalent to "%I:%M:%S %p". [tm_sec, tm_min, tm_hour]
`%R`
  The 24-hour time, to the minute. Equivalent to "%H:%M". [tm_min, tm_hour]
`%S`
  The second, formatted with two digits (from ``00`' to ``60`'). The value 60 accounts for the occasional leap second. [tm_sec]
`%t`
  A tab character (``\t`').
`%T`
  The 24-hour time, to the second. Equivalent to "%H:%M:%S". [tm_sec, tm_min, tm_hour]
`%u`
  The weekday as a number, 1-based from Monday (from ``1`' to ``7`'). [tm_wday]
`%U`
  The week number, where weeks start on Sunday, week 1 contains the first Sunday in a year, and earlier days are in week 0. Formatted with two digits (from ``00`' to ``53`'). See also `%W`. [tm_wday, tm_yday]
`%V`
  The week number, where weeks start on Monday, week 1 contains January 4th, and earlier days are in the previous year. Formatted with two digits (from ``01`' to ``53`'). See also `%G`. [tm_year, tm_wday, tm_yday]
`%w`
  The weekday as a number, 0-based from Sunday (from ``0`' to ``6`'). [tm_wday]
`%W`
  The week number, where weeks start on Monday, week 1 contains the first Monday in a year, and earlier days are in week 0. Formatted with two digits (from ``00`' to ``53`'). [tm_wday, tm_yday]
`%x`
  A string representing the complete date, equivalent to "%m/%d/%y". [tm_mon, tm_mday, tm_year]
`%X`
  A string representing the full time of day (hours, minutes, and seconds), equivalent to "%H:%M:%S". [tm_sec, tm_min, tm_hour]
`%y`
  The last two digits of the year (from ``00`' to ``99`'). [tm_year] (Implementation interpretation: always positive, even for negative years.)
`%Y`
  The full year, equivalent to `%C%y`. It will always have at least four characters, but may have more. The year is accurate even when tm_year added to the offset of 1900 overflows an int. [tm_year]
`%z`
  The offset from UTC. The format consists of a sign (negative is west of Greewich), two characters for hour, then two characters for minutes (-hhmm or +hhmm). If tm_isdst is negative, the offset is unknown and no output is generated; if it is zero, the offset is the standard offset for the current time zone; and if it is positive, the offset is the daylight savings offset for the current timezone. The offset is determined from the TZ environment variable, as if by calling tzset(). [tm_isdst]
`%Z`
  The time zone name. If tm_isdst is negative, no output is generated. Otherwise, the time zone name is based on the TZ environment variable, as if by calling tzset(). [tm_isdst]
`%%`
  A single character, ``%`'.

Returns
When the formatted time takes up no more than maxsize characters, the result is the length of the formatted string. Otherwise, if the formatting operation was abandoned due to lack of room, the result is `0`, and the string starting at s corresponds to just those parts of `*`format that could be completely filled in within the maxsize limit.

Portability
ANSI C requires `strftime`, but does not specify the contents of `*`s when the formatted string would require more than maxsize characters. Unrecognized specifiers and fields of `timp` that are out of range cause undefined results. Since some formats expand to 0 bytes, it is wise to set `*`s to a nonzero value beforehand to distinguish between failure and an empty string. This implementation does not support `s` being NULL, nor overlapping `s` and `format`.

`strftime` requires no supporting OS subroutines.

Bugs
`strftime` ignores the LC_TIME category of the current locale, hard-coding the "C" locale settings.