Assuming we don't want to maintain a set of per-platform macros, we need to use an existing, standard format specifier. There isn't one that takes a time_t. So we have to cast time_t to a type big enough to contain it, use the format specifier for that type, and we want this to be as cross-platform as possible, i.e. we use the oldest, most widely-supported type which can definitely hold at least 64 bits and has a standard printf specifier available. i.e. "long long", exactly as in your example code.

So that's why we prefer "long long" rather than "int64_t", which I thought was your original question.