Anyone who's been programming for more than a year knows, or should know, that you don't do time arithmetic directly on the date representation. You convert times to some form that is easy to do arithmetic on, like seconds from 1970-1-1 00:00:00 UTC (the Unix epoch) or 1900-1-1 00:00:00 UTC (the Common Lisp epoch), do all your arithmetic on that, then convert back.

So to add a year, all you have to do is add 365.25 * 24 * 60 * 60.

This was a stunningly stupid, n00b mistake.