While fixing the error when the year 9999 is reached is a good thing,
have you considered adding a <meta name="robots" content="noindex,
nofollow"> so that the search engines* will avoid wasting time and
resources to go through ~8000 years of links needlessly?

If you think this is reasonable I can add the meta tag to the
_generic.calendar.html page.

* Note that this won't prevent all the robots to follow the links, but
it's still better than nothing.