Given two (possibly unordered) binary timestamps, returns a
    single UTC time interval whose inaccuracy spans the two input
    binary timestamps.

    Format

      #include <utc.h>

      int utc_spantime(*result, *utc1, *utc2)

         utc_t *result;
         const utc_t *utc1;
         const utc_t *utc2;