In <https://www.opengroup.org/sophocles/show_mail.tpl?source=L&listname=austin-g...>, Eric Blake <ebb9@byu.net> writes:

this means that %Y is not equivalent to %C%y, since %C is documented to be exactly two digits

This topic came up around 2004-08-20 in the tz mailing list. (Please see <http://www.twinsun.com/tz/tz-link.htm> for info about the list; it is archived in <ftp://elsie.nci.nih.gov/pub/tzarchive.gz>.) The consensus was as follows (presumably a defect report could also be submitted to POSIX, as POSIX can place requirements on strftime that are over and above what ISO C requires): * The ISO C standard is broken in this area; the standardizers did not think about the problem. (One of those involved in the tz discussion was also involved with the ISO C effort.) * A Defect Report should be submitted. (Nobody has yet done so, to my knowledge.) * Best practice (which is not currently required by any standard) is to have the following behavior, originally suggested by Arthur David Olson: - %Y always generates the mathematically-correct result, even if the expression "tm.tm_year + 1900" would overflow. - %Y is always equivalent to %C%y. - %y is always in the range [00,99]. - %C is in the range [00,99] when possible. - %C is two characters long when possible. Here are some examples: %Y %C %y 10000 100 00 9999 99 99 0999 09 99 0099 00 99 0001 00 01 0000 00 00 -001 -0 01 -099 -0 99 -999 -9 99 -1000 -10 00 -10000 -100 00 %Y %C %y -2147481749 -21474817 49 2147485547 21474855 47 Note that the last example overflows 32-bit 'int', but it is the best-practice behavior even on 32-bit-int hosts. * The tz library <ftp://elsie.nci.nih.gov/pub/> now has the best-practice behavior suggested above.