docs: public int compareTo(ReadableInstant other) {
          if (this == other) {
              return 0;
          }
          
          long otherMillis = other.getMillis();
          long thisMillis = getMillis();
          
          // cannot do (thisMillis - otherMillis) as can overflow
          if (thisMillis == otherMillis) {
              return 0;
          }
          if (thisMillis < otherMillis) {
              return -1;
          } else {
              return 1;
          }
      }