I have reviewed the latest patch, and it looks good to me. There are tests for the tricky conversions around Jan 1, and the docs are brief and succinct. Until the full set of new c99 strftime directives are supported, I think it's overkill to include a lecture about the origin of these new directives and their support in the underlying OS.

There are a number of foot-shooting possibilities when parsing date strings with the directives supported by strptime(), but at least this patch does not make it worse. It validates the input nicely when using the new directives and prints useful error messages it input is ambiguous.

I'm not a committer, but I approve of the patch as-is.