Computes the difference between two binary timestamps that
    express either an absolute time and a relative time, two relative
    times, or two absolute times.

    Format

      #include <utc.h>

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

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