Casting between structs?

typedef struct {
    unsigned int hi, lo;
} int64;

int64 int64_add(int64 x, int64 y);

#define int64_add(x, y) ((x) + (y))

typedef int64 mytime;

#define mytime_add(x, y) (mytime) int64_add((int64) (x), (int64) (y))

typedef struct int64 int64;

struct int64
{
    unsigned int hi, lo;
};

typedef int64 mytime;

int64
add_int64(int64 a, int64 b)
{
    int64 c;
    /* I know that is wrong */
    c.hi = a.hi + b.hi;
    c.lo = a.lo + b.lo;

    return c;
}

int
main(void)
{
    mytime a = {1, 2};
    mytime b = {3, 4};
    mytime c;

    c = add_int64(a, b);

    return 0;
}

#define mytime_add(x, y) int64_add((x), (y))

mytime mytime_add(mytime x, mytime y) { return int64_add(x, y); }

#define mytime int64