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   strftime.3p    ( 3 )

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Обоснование (Rationale)

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 <hyphen-minus> 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 <hyphen-minus> character ('-') 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().