Getting a date from ISO week /weekday is not possible with the %W and %w directives. ISO week numbers and normal week numbers are calculated differently (see my libc qoute in the original post). Also, using %w instead of %u would be ambiguous, since %w weeks start on Sunday, while %u start with Monday.

I agree that this should be implemented in strptime(), to follow libc strptime(). datetime() has a clear constructor while strptime() has all the whacky cases.