The C version of Unix was written for a 16-bit processor (pdp-11). The C compiler simulated 32-bit operations but nothing bigger. 64-bit operations only got wide spread way later when 16-bit systems were no longer relevant and 32-bit systems got 'long long'. Note that POSIX allows time_t to be 64-bit. And as far as I know, that's what OpenBSD does.
