I prefer signed time types, because you frequently subtract them. If you use unsigned, then you have to cast the result to signed every time:

  int64_t elapsed = (int64_t)(t1 - t0);

And it's very easy to cause disaster with:

  if ((t1 - t0) > 50) ...

which also succeeds if t1<t0.

While it's theoretically possible, using all 64 bits is tricky and very hard to test.