localtime isn't implemented in Perl as a function. It's actually a pp code. As of Perl 5.8.0, see pp_sys.c. That actually won't help you, either, because it takes and returns internal Perl data structures.

All of this is mostly academic, though, because there's already a C localtime call, if you really need it.

I'm not good at XS, but I'd break my code into smaller chunks and do as little in C as possible; if you find yourself needing to call internal Perl pp codes, you could probably refactor your code down more sooner than you could figure out how to fake up what the pp code actually needs. (See again my disclaimer about not being good at XS, though.)