To go further: define an API, write tests against that API, and then do a pass of dead-code analysis on the resulting library-plus-test-suite. Any private functions left uncalled by your public API can just be removed!
To counter that I'd say you should have a test case to cover the leap year handling is working as expected. If you aren't testing that since it wasn't in the spec, than why would you have the code at all?
