In C, using: struct tm tm_struct = { 0 } ;, the rest of the struct including padding is initialized to zero bits.

Yes, as far as I believe, this should always work in standard C89/C99. In the char array case, it works due to the brace elision rule in aggregates: assuming

    struct foo { char bar[3]; };

These declarations are equivalent:

    struct foo f1 = { 0, };
    struct foo f2 = { { 0, } };