We have a NetBSD 1.2 machine with uptime of 388 days.  ruptime reports
the uptime as "??":

        if (tval < 0 || tval > 365*24*60*60) {
                (void)sprintf(resbuf, "   %s ??:??", updown);
                return(resbuf);
        }

Why? :-)

  -jm