OpenBSD's man page for strptime(3) states in the Standards section: The `%G', `%g', `%s', `%u', `%V', `%Y', and `%Z' conversion specifications are extensions. However, %Y is not an extension. It is part of the standard. There are also extensions not listed there that should be: %F, %k, %l (lowercase L), %z
https://man.openbsd.org/strptime.3 https://pubs.opengroup.org/onlinepubs/9699919799/functions/strptime.html However, my email may not be fully meaningful. Also in the Standards section of the man page is: The strptime() function conforms to X/Open Portability Guide Issue 4, Version 2 ("XPG4.2"). I instead looked at the most recent version of that standard (XPG7) since I don't know if the old version specified there is still available online. In the past I've been able to find XPG7, XPG6, XPG5, and XPG2 online. Only XPG7 and XPG6 are easy to find.
