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   strftime.3p    ( 3 )

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Использование в приложениях (Application usage)

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 or POSIX 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.