Pullin' my hair out... arrrgh

I need a script to execute every 18 hours w/o having to change the crontab daily, if possible. I have tried sleeping/recalling script but, of course, one has to then kill a bunch of pids for older executions of the same script ...but at least not daily, I don't suppose. Is there a cleaner way of doing this? Thanks! -- ...CH "The more they over-think the plumbing, the easier it is to stop up the drain." Scotty