you might call this a gotcha of this obscure standard called UNIX/POSIX, although...

> %G The ISO 8601 week-based year (see NOTES) with century as a decimal number. The 4-digit year corresponding to the ISO week number (see %V). This has the same format and value as %Y, except that if the ISO week number belongs to the previous or next year, that year is used instead. (TZ)

I think that's pretty clear, so its actually just a case of not reading / bothering to understand the documentation. I think if you really want to bulletproof a system against incompetency we should probably not start with C.