I'd like to work with human-readable date formats - ie, allow the user to select the date format for input and output using a human-readable string such as 'yyyy-mm-dd'.

What user where in what context?

I think that is too ambiguous, its better to provide the human a list of examples, or an interactive program.... or a way to contact a programmer

OTOH see DateTime::Format::Natural :)

use a non-human-friendly %X format

They're made for programmers, who are supposed to make life easier for "humans" :)