Perhaps it could be mentioned more clearly in the datetime documentation that apart from the directives of the C89 standard, you can also use those that the underlying libc strftime() supports.

The following sentence from https://docs.python.org/3/library/time.html#time.strftime describes it well in my opinion:
"Additional directives may be supported on certain platforms, but only the ones listed here have a meaning standardized by ANSI C."

Perhaps a reference to "man strftime" would also be helpful.