Converts a tm structure that expresses relative time to a
    relative binary timestamp.

    Format

      #include <utc.h>

      int utc_mkreltime(*utc, *timetm, tns, *inacctm, ins)

         utc_t *utc;
         const struct tm *timetm;
         long tns;
         const struct tm *inacctm;
         long ins;