In the tzcode2007h.tar.gz file, there is (at least) one function that is defined in prototype notation:

static time_t
hunt(char *name, time_t lot, time_t hit)

It seems to me that the code assumes that there is a 64-bit integer type (int_fast64_t (function detzcode64()), though the code goes through some machinations to find a 64-bit integer and doesn't outright assume C99 compliance.

Is there actually a good reason not to write the whole library with ISO prototypes?  And lose the historically necessary but now unnecessary P macro for declaring functions with (or without) prototypes.  Which platform(s) do not provide a C compiler that is capable of handling prototypes?  I know of none on the major platforms...Unix, Linux, Windows (though maybe the bundled C compiler on HP-UX is still unaware, but that is intended for linking kernels, not program development).